Fix a NULL dereference
Fix a NULL dereference

Access to field 'zone' results in a dereference of a null pointer (loaded from variable 'prev_preset'), if `size` is negative. Problem is: Parameter `size` is `chunk.size` and should be unsigned.

FluidSynth/fluidsynth,FluidSynth/fluidsynth,FluidSynth/fluidsynth,FluidSynth/fluidsynth