THRIFT-5854: Move the checkReadBytesAvailable check before allocation

[bandle]

Affects: TCompactProtocol::readBinary

We allocated the string first and only checked afterward whether this allocation would be larger than MaxMessageSize allows. Since it throws, we throw away the buffer and the read, so we should check it earlier.

Furthermore, we check for varint_size + strlen instead of only strlen. However, since we can read varint using borrow and consume, this already decreases the remainingMessageSize by varint_size. Thus it can falsely trigger MaxMessageSize.

The tests for TCompactProtocol now use values closer to the egde case ana also test for successfull reads instead of only checking for exceptions.

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