avconv: add an assert to silence an uninitialized variable warning.

The warning silenced was:
avconv.c: In function ‘opt_output_file’:
avconv.c:3380:21: warning: ‘meta_out’ may be used uninitialized in this function [-Wuninitialized]
avconv.c:3315:20: note: ‘meta_out’ was declared here

avconv.c

@@ -3350,6 +3350,7 @@ static int copy_metadata(char *outspec, char *inspec, AVFormatContext *oc, AVFor
             METADATA_CHECK_INDEX(index, context->nb_programs, "program")\
             meta = &context->programs[index]->metadata;\
             break;\
+        default: av_assert0(0);\
         }\
 
     SET_DICT(type_in, meta_in, ic, idx_in);