Fix NRE with UnicodeEncoding when target is an empty span (#97950)

* Fix NRE with UnicodeEncoding when target is an empty span

* Faster check

* Same fix for UTF32Encoding

* Move test to NegativeEncodingTests

* Apply same fixes in System.Text.Encoding.Codepages

---------

Co-authored-by: Stephen Toub <stoub@microsoft.com>

Author: manandre

src/libraries/System.Private.CoreLib/src/System/Text/Encoding.cs

@@ -1297,7 +1297,7 @@ internal unsafe bool AddChar(char ch, int numBytes)
                     {
                         // Throw maybe
                         _bytes -= numBytes;                                        // Didn't encode these bytes
-                        _enc.ThrowCharsOverflow(_decoder, _bytes <= _byteStart);    // Throw?
+                        _enc.ThrowCharsOverflow(_decoder, _chars == _charStart);    // Throw?
                         return false;                                           // No throw, but no store either
                     }
 
@@ -1316,7 +1316,7 @@ internal unsafe bool AddChar(char ch1, char ch2, int numBytes)
                 {
                     // Throw maybe
                     _bytes -= numBytes;                                        // Didn't encode these bytes
-                    _enc.ThrowCharsOverflow(_decoder, _bytes <= _byteStart);    // Throw?
+                    _enc.ThrowCharsOverflow(_decoder, _chars == _charStart);    // Throw?
                     return false;                                           // No throw, but no store either
                 }
                 return AddChar(ch1, numBytes) && AddChar(ch2, numBytes);

src/libraries/System.Private.CoreLib/src/System/Text/UTF32Encoding.cs

@@ -934,7 +934,7 @@ internal override unsafe int GetChars(byte* bytes, int byteCount,
                 if (iChar >= 0x10000)
                 {
                     // Surrogates take 2
-                    if (chars >= charEnd - 1)
+                    if (charEnd - chars < 2)
                     {
                         // Throwing or stopping
                         // We either read enough bytes for bytes-=4 to work, or we're

src/libraries/System.Private.CoreLib/src/System/Text/UnicodeEncoding.cs

@@ -1535,7 +1535,7 @@ internal sealed override unsafe int GetChars(
                     }
 
                     // Valid surrogate pair, add our lastChar (will need 2 chars)
-                    if (chars >= charEnd - 1)
+                    if (charEnd - chars < 2)
                     {
                         // couldn't find room for this surrogate pair
                         // We either advanced bytes or chars should == charStart and throw below

src/libraries/System.Runtime/tests/System.Text.Encoding.Tests/NegativeEncodingTests.cs

@@ -623,6 +623,9 @@ void VerifyOutParams()
             // Chars does not have enough space
             AssertExtensions.Throws<ArgumentException>("chars", () => decoder.Convert(new byte[4], 0, 4, new char[0], 0, 0, flush, out charsUsed, out bytesUsed, out completed));
             VerifyOutParams();
+
+            AssertExtensions.Throws<ArgumentException>("chars", () => decoder.Convert(encoding.GetBytes("\uD800\uDC00".ToCharArray()).AsSpan(), new char[0].AsSpan(), flush, out charsUsed, out bytesUsed, out completed));
+            VerifyOutParams();
         }
 
         [Theory]

src/libraries/System.Text.Encoding.CodePages/src/System/Text/EncodingCharBuffer.cs

@@ -53,7 +53,7 @@ internal unsafe bool AddChar(char ch, int numBytes)
                 {
                     // Throw maybe
                     _bytes -= numBytes;                                        // Didn't encode these bytes
-                    _enc.ThrowCharsOverflow(_decoder, _bytes <= _byteStart);    // Throw?
+                    _enc.ThrowCharsOverflow(_decoder, _chars == _charStart);    // Throw?
                     return false;                                           // No throw, but no store either
                 }
 
@@ -72,11 +72,11 @@ internal unsafe bool AddChar(char ch)
         internal unsafe bool AddChar(char ch1, char ch2, int numBytes)
         {
             // Need room for 2 chars
-            if (_chars >= _charEnd - 1)
+            if (_charEnd - _chars < 2)
             {
                 // Throw maybe
                 _bytes -= numBytes;                                        // Didn't encode these bytes
-                _enc.ThrowCharsOverflow(_decoder, _bytes <= _byteStart);    // Throw?
+                _enc.ThrowCharsOverflow(_decoder, _chars == _charStart);    // Throw?
                 return false;                                           // No throw, but no store either
             }
             return AddChar(ch1, numBytes) && AddChar(ch2, numBytes);