fix panics in protoparse identified by oss-fuzz #412
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…t collided with the various constants for known token types
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https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=34232
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=34238
Both bugs had the same root cause: unicode chars with very high code points that collided with the various constants for known token types.
So now the lexer is smart enough to limit the symbol type value for such runes to less than 256. The smallest constant is 57346, so plenty of headroom in between to avoid collisions. If it sees a symbol with a greater code point, it will now report an error "invalid character".
This should be fine since this handling is only for stand-alone runes/punctuation. Protobuf does not allow such symbols as part of identifiers nor as any allowed punctuation or operator. Unicode points like this are only valid in a proto source file inside of a string literal. So if we come across a lone rune with such a value (not in a string literal), then it is definitely a syntax error.