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/mmsh.c

@@ -208,7 +208,6 @@ static int get_http_header_data(MMSHContext *mmsh)
             }
         }
     }
-    return 0;
 }
 
 static int mmsh_open(URLContext *h, const char *uri, int flags)