avconv: remove bogus warning when using avconv -h without parameter

avconv_opt.c

@@ -1792,7 +1792,7 @@ void show_help_default(const char *opt, const char *arg)
     const int per_file = OPT_SPEC | OPT_OFFSET | OPT_PERFILE;
     int show_advanced = 0, show_avoptions = 0;
 
-    if (opt) {
+    if (opt && *opt) {
         if (!strcmp(opt, "long"))
             show_advanced = 1;
         else if (!strcmp(opt, "full"))