avcodec/dcadec: fix a -Wsometimes-unitialized on clang

This fixes a genuine -Wsometimes-unitialized reported on clang, e.g
http://fate.ffmpeg.org/log.cgi?time=20150918181527&log=compile&slot=x86_64-darwin-clang-polly-vectorize-stripmine-3.7.
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/dcadec.c

@@ -1456,7 +1456,7 @@ static float dca_dmix_code(unsigned code)
 static int scan_for_extensions(AVCodecContext *avctx)
 {
     DCAContext *s = avctx->priv_data;
-    int core_ss_end, ret;
+    int core_ss_end, ret = 0;
 
     core_ss_end = FFMIN(s->frame_size, s->dca_buffer_size) * 8;