Avoid division by zero

Based on clang-scan report http://permalink.gmane.org/gmane.comp.video.ffmpeg.devel/107290

Originally committed as revision 22795 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/zmbv.c

@@ -434,6 +434,7 @@ static int decode_frame(AVCodecContext *avctx, void *data, int *data_size, AVPac
         }
         if(c->bw == 0 || c->bh == 0) {
             av_log(avctx, AV_LOG_ERROR, "Unsupported block size %ix%i\n", c->bw, c->bh);
+            return -1;
         }
         if(c->comp != 0 && c->comp != 1) {
             av_log(avctx, AV_LOG_ERROR, "Unsupported compression type %i\n", c->comp);