Fixed string exif value corner case exception

[br3aker]

Prerequisites

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• I have provided test coverage for my change (where applicable)

Description

Closes #2087.

Given image from linked issue contains empty user comment tag which is written last. Allocated buffer is passed as Span which is sliced during different exif values rendering to the given buffer. This exact situation led to copying zero bytes to zero-length span aaaaand looks like .net treats empty spans with valid memory pointer as null pointer thus mentioned ArgumentNullException.

Fix is fairly simple: use spans instead of working with raw pointers without any fixed statements (love this performance side effect) and unsafe stuff, Encoding.GetBytes(ReadOnlySpan, Span) respects empty spans.

P.S.
Waiting for issue image usage approval from the issue author.

[br3aker]

@JimBobSquarePants can you please advise where should I place the image from this issue?
I don't see any similar tests.

[antonfirsov]

@br3aker I think adding yet another entry to BaselineTestJpegs or ProgressiveTestJpegs is good enough, we have a bunch of Exif regression cases there already.

[JimBobSquarePants]

Yep, just add it to the collection there

[br3aker]

Problem is that this image decodes correctly, it throws on encoding during writing exif profile.

[JimBobSquarePants]

It’s cool, we did the same here.

#2059

[br3aker]

Done, hope I got your idea right.

[JimBobSquarePants]

Looks perfect. Thanks 👍