CC Mode: Fix a bug in yesterday's patch

* lisp/progmodes/cc-mode.el (c-before-change-check-unbalanced-strings): Check
the language has multi-line strings before calling
c-ml-string-opener-at-or-around-point.

lisp/progmodes/cc-mode.el

@@ -1526,6 +1526,7 @@ Note that the style variables are always made local to the buffer."
     (unless
 	(or (and
 	     ;; Don't set c-new-BEG/END if we're in an ml string.
+	     c-ml-string-opener-re
 	     (eq beg-literal-type 'string)
 	     (c-ml-string-opener-at-or-around-point (car beg-limits)))
 	    (and c-multiline-string-start-char