Misc SILK fixes:

- compile warning in opus_decoder.c
- decoder state reduced by ~3 kB (by merging buffers, as Tim suggested)
- some minor decoder optimizations (only the PLC is non-bit exact, so should be ok)

src/opus_decoder.c

@@ -418,7 +418,7 @@ static int opus_decode_frame(OpusDecoder *st, const unsigned char *data,
             pcm[i] = SAT16(pcm[i] + pcm_silk[i]);
 #else
         for (i=0;i<frame_size*st->channels;i++)
-            pcm[i] = pcm[i] + (1./32768.)*pcm_silk[i];
+            pcm[i] = pcm[i] + (opus_val16)((1./32768.)*pcm_silk[i]);
 #endif
     }