delete source of buffer overrun on Windows

The Windows Qt build just got crash after crash because of this

Just to explain `memset(stereoout + 2 * 2 * numFrames, 0, 2 * 2 * (numSamples - numFrames));` assumed that `stereoout` was a `char *` but it's a `short *` so it writes outside of the buffer `stereoout`

The problem was that on windows the full buffersize was never passed, so this error was masked by the fact that the buffer is way larger than in needs to be by default, but in the Qt build we pass the actual size.