movenc: Add a warning message if conflicting options have been specified

Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/movenc.c

@@ -3141,8 +3141,11 @@ static int mov_write_header(AVFormatContext *s)
     /* faststart: moov at the beginning of the file, if supported */
     if (mov->flags & FF_MOV_FLAG_FASTSTART) {
         if ((mov->flags & FF_MOV_FLAG_FRAGMENT) ||
-            (s->flags & AVFMT_FLAG_CUSTOM_IO))
+            (s->flags & AVFMT_FLAG_CUSTOM_IO)) {
+            av_log(s, AV_LOG_WARNING, "The faststart flag is incompatible "
+                   "with fragmentation and custom IO, disabling faststart\n");
             mov->flags &= ~FF_MOV_FLAG_FASTSTART;
+        }
     }
 
     /* Non-seekable output is ok if using fragmentation. If ism_lookahead