Brotli compress_helper truncates compressed output when input buffer is larger than 1.5MB

A bug in the Brotli `compress_helper`'s `compress()` method results in premature termination of the compressed data when the final `getn()` or `acquire()` on the input stream's `streambuf `returns data exceeding the Brotli library's internal buffer size (about 1.5MB by default).