fftools/cmdutils: Use av_strstart instead of strncmp

It makes the intent clearer and avoids searching for a delimiter
in advance.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

fftools/cmdutils.c

@@ -215,11 +215,9 @@ void show_help_children(const AVClass *class, int flags)
 
 static const OptionDef *find_option(const OptionDef *po, const char *name)
 {
-    const char *p = strchr(name, ':');
-    int len = p ? p - name : strlen(name);
-
     while (po->name) {
-        if (!strncmp(name, po->name, len) && strlen(po->name) == len)
+        const char *end;
+        if (av_strstart(name, po->name, &end) && (!*end || *end == ':'))
             break;
         po++;
     }