Remove return statements following infinite loops without break

These statements cannot be reached and are thus not needed.
This removes a number of compiler warnings.

Signed-off-by: Mans Rullgard <mans@mansr.com>

libavformat/mm.c

@@ -184,8 +184,6 @@ static int read_packet(AVFormatContext *s,
             avio_skip(pb, length);
         }
     }
-
-    return 0;
 }
 
 AVInputFormat ff_mm_demuxer = {