adpcm: Check for channels to be a non-zero integer

channels would be 0 sometimes and would cause floating point exception
Fixes bugzilla #124

Signed-off-by: Justin Ruggles <justin.ruggles@gmail.com>
(cherry picked from commit e614fac2e6)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/adpcm.c

@@ -101,8 +101,9 @@ static av_cold int adpcm_decode_init(AVCodecContext * avctx)
         max_channels = 6;
         break;
     }
-    if(avctx->channels > max_channels){
-        return -1;
+    if (avctx->channels <= 0 || avctx->channels > max_channels) {
+        av_log(avctx, AV_LOG_ERROR, "Invalid number of channels\n");
+        return AVERROR(EINVAL);
     }
 
     switch(avctx->codec->id) {