grep: fix colouring of matches with zero length

If a zero-length match is encountered, break out of loop and show the rest
of the line uncoloured.  Otherwise we'd be looping forever, trying to make
progress by advancing the pointer by zero characters.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

grep.c

@@ -500,6 +500,8 @@ static void show_line(struct grep_opt *opt, char *bol, char *eol,
 
 		*eol = '\0';
 		while (next_match(opt, bol, eol, ctx, &match, eflags)) {
+			if (match.rm_so == match.rm_eo)
+				break;
 			printf("%.*s%s%.*s%s",
 			       (int)match.rm_so, bol,
 			       opt->color_match,