C standards say that "char" may either be a "signed char" or "unsigned char" but that it is up to the compilers implementation or the platform which is followed. Some non x86 platforms, including PowerPC and ARM, treat unspecified chars as unsigned chars, so it is necessary to explicitly declare them as "signed char" (or to compile with "--signed_chars").
This fix ensures that fluid synth's sample's pitchadj value are correctly read as signed.
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