avconv: fix copying per-stream metadata.

It is handled separately from other types because it uses stream
specifiers and currently that triggers an assert in SET_DICT.

CC:libav-stable@libav.org

avconv_opt.c

@@ -351,6 +351,8 @@ 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;\
+        case 's':\
+            break; /* handled separately below */ \
         default: av_assert0(0);\
         }\