patch 8.0.0643: when a pattern search is slow Vim becomes unusable

Problem:    When 'hlsearch' is set and matching with the last search pattern
            is very slow, Vim becomes unusable.  Cannot quit search by
            pressing CTRL-C.
Solution:   When the search times out set a flag and don't try again.  Check
            for timeout and CTRL-C in NFA loop that adds states.

src/screen.c

@@ -7800,20 +7800,21 @@ next_search_hl(
 	    int regprog_is_copy = (shl != &search_hl && cur != NULL
 				&& shl == &cur->hl
 				&& cur->match.regprog == cur->hl.rm.regprog);
+	    int timed_out = FALSE;
 
 	    nmatched = vim_regexec_multi(&shl->rm, win, shl->buf, lnum,
 		    matchcol,
 #ifdef FEAT_RELTIME
-		    &(shl->tm)
+		    &(shl->tm), &timed_out
 #else
-		    NULL
+		    NULL, NULL
 #endif
 		    );
 	    /* Copy the regprog, in case it got freed and recompiled. */
 	    if (regprog_is_copy)
 		cur->match.regprog = cur->hl.rm.regprog;
 
-	    if (called_emsg || got_int)
+	    if (called_emsg || got_int || timed_out)
 	    {
 		/* Error while handling regexp: stop using this regexp. */
 		if (shl == &search_hl)