fix phase modulation not to treat phase modulation data as unsigned before multiplying

synth_waveform.cpp

 		bp = moddata->data;
 		for (i=0; i < AUDIO_BLOCK_SAMPLES; i++) {
 			// more than +/- 180 deg shift by 32 bit overflow of "n"
-			uint32_t n = (uint16_t)(*bp++) * modulation_factor;
+			uint32_t n = (*bp++) * modulation_factor;
 			phasedata[i] = ph + n;
 			ph += inc;
 		}