ffmpeg: Fix assert failure

Fixes Ticket1674
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg_opt.c

@@ -443,6 +443,8 @@ static int copy_metadata(char *outspec, char *inspec, AVFormatContext *oc, AVFor
             meta = &context->programs[index]->metadata;\
             break;\
         default: av_assert0(0);\
+        case 's':\
+            break;\
         }\
 
     SET_DICT(type_in, meta_in, ic, idx_in);