Replace "signed short" typecast with "int16_t" as suggested by

Mans Rullgard. Value at the right side is 16 bit length signed.
We can not know for sure that short is in fact 16 bits, even if
this virtually always is the case.

Originally committed as revision 10666 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/adpcm.c

@@ -1184,7 +1184,7 @@ static int adpcm_decode_frame(AVCodecContext *avctx,
         break;
     case CODEC_ID_ADPCM_IMA_AMV:
     case CODEC_ID_ADPCM_IMA_SMJPEG:
-        c->status[0].predictor = (signed short)bytestream_get_le16(&src);
+        c->status[0].predictor = (int16_t)bytestream_get_le16(&src);
         c->status[0].step_index = bytestream_get_le16(&src);
 
         if (avctx->codec->id == CODEC_ID_ADPCM_IMA_AMV)