js-mode: Fix indent problem after a regexp

* lisp/progmodes/js.el (js--looking-at-operator-p): Check that the
slash is not ending a regexp (bug#24854).

lisp/progmodes/js.el

@@ -1756,6 +1756,10 @@ This performs fontification according to `js--class-styles'."
              (save-excursion
                (and (js--re-search-backward "[?:{]\\|\\_<case\\_>" nil t)
                     (eq (char-after) ??))))
+         (not (and
+               (eq (char-after) ?/)
+               (save-excursion
+                 (eq (nth 3 (syntax-ppss)) ?/))))
          (not (and
                (eq (char-after) ?*)
                ;; Generator method (possibly using computed property).