AudioEffectPitchShift: Prevent negative size memset (GCC warning)

[akien-mga]

Fixes a warning when using it for memset (from #88504):

servers/audio/effects/audio_effect_pitch_shift.cpp: In member function 'PitchShift.constprop':
servers/audio/effects/audio_effect_pitch_shift.cpp:163:31: warning: 'memset' specified bound between 18446744065119617024 and 18446744073709551608 exceeds maximum object size 9223372036854775807 [-Wstringop-overflow=]
  163 |                         memset(gSynMagn, 0, fftFrameSize*sizeof(float));
      |                               ^
servers/audio/effects/audio_effect_pitch_shift.cpp:164:31: warning: 'memset' specified bound between 18446744065119617024 and 18446744073709551608 exceeds maximum object size 9223372036854775807 [-Wstringop-overflow=]
  164 |                         memset(gSynFreq, 0, fftFrameSize*sizeof(float));
      |                               ^

This is "thirdparty" code part of our own Godot code, so I just worked around the problem where it appears, instead of refactoring the whole function to properly make use of unsigned types throughout when signed results aren't needed.