cmdutils: Exit in case of faulty stream specifiers

Avoids printing confusing and unrelated errors
Fixes Ticket2772
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

cmdutils.c

@@ -1997,7 +1997,7 @@ AVDictionary *filter_codec_opts(AVDictionary *opts, enum AVCodecID codec_id,
             switch (check_stream_specifier(s, st, p + 1)) {
             case  1: *p = 0; break;
             case  0:         continue;
-            default:         return NULL;
+            default:         exit_program(1);
             }
 
         if (av_opt_find(&cc, t->key, NULL, flags, AV_OPT_SEARCH_FAKE_OBJ) ||