memory: avoid a leak when a string bind specifies an unknown menu

(It is a harmless leak, but LeakSanitizer is loud when it complains.)

After having determined that there is a menu name, first check that
it is valid, before processing the string or the function name.

This fixes https://savannah.gnu.org/bugs/?62991.

Problem existed since version 2.9.4, since string binds were introduced.

src/rcfile.c

@@ -787,6 +787,12 @@ void parse_binding(char *ptr, bool dobind)
 		goto free_things;
 	}
 
+	menu = name_to_menu(menuptr);
+	if (menu < 1) {
+		jot_error(N_("Cannot map name \"%s\" to a menu"), menuptr);
+		goto free_things;
+	}
+
 	if (dobind) {
 		/* If the thing to bind starts with a double quote, it is a string,
 		 * otherwise it is the name of a function. */
@@ -806,12 +812,6 @@ void parse_binding(char *ptr, bool dobind)
 		}
 	}
 
-	menu = name_to_menu(menuptr);
-	if (menu < 1) {
-		jot_error(N_("Cannot map name \"%s\" to a menu"), menuptr);
-		goto free_things;
-	}
-
 	/* Wipe the given shortcut from the given menu. */
 	for (keystruct *s = sclist; s != NULL; s = s->next)
 		if ((s->menus & menu) && s->keycode == keycode)