Detect invalid unicode class specification

[sjamesr]

Previously, the parser would unconditionally attempt to read the unicode
class spec after a \p or \P, even if the pattern is truncated and
invalid.

Fixes issue #72.

[coveralls]

Pull Request Test Coverage Report for Build 225

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.07%) to 93.966%

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Files with Coverage Reduction	New Missed Lines	%
/home/travis/build/google/re2j/java/com/google/re2j/Parser.java	2	95.38%

Totals
Change from base Build 222:	0.07%
Covered Lines:	2943
Relevant Lines:	3132

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💛 - Coveralls