Fix possible out-of-bounds array access

If match is "", match[-1] is accessed.  Let pathspec_matches return 1 in that
case indicating that "" matches everything.

Incidently this fixes git-grep'ing in ".".

Signed-off-by: Uwe Zeisberger <Uwe_Zeisberger@digi.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

builtin-grep.c

@@ -29,10 +29,11 @@ static int pathspec_matches(const char **paths, const char *name)
 		int matchlen = strlen(match);
 		const char *cp, *meta;
 
-		if ((matchlen <= namelen) &&
-		    !strncmp(name, match, matchlen) &&
-		    (match[matchlen-1] == '/' ||
-		     name[matchlen] == '\0' || name[matchlen] == '/'))
+		if (!matchlen ||
+		    ((matchlen <= namelen) &&
+		     !strncmp(name, match, matchlen) &&
+		     (match[matchlen-1] == '/' ||
+		      name[matchlen] == '\0' || name[matchlen] == '/')))
 			return 1;
 		if (!fnmatch(match, name, 0))
 			return 1;