watchfrr: fix SA warning

`valid_command` now causes static analyzer complaints since it no
longer assumes `optarg` is non-NULL. If this was the case then
`valid_command` would return `false` (or 0) because it would mean the
string is empty and doesn't contain the '%s' it expects.

Signed-off-by: Rafael Zalamena <rzalamena@opensourcerouting.org>

watchfrr/watchfrr.c

@@ -1080,6 +1080,9 @@ static int valid_command(const char *cmd)
 {
 	char *p;
 
+	if (cmd == NULL)
+		return 0;
+
 	return ((p = strchr(cmd, '%')) != NULL) && (*(p + 1) == 's')
 	       && !strchr(p + 1, '%');
 }