attr: fix segfault in gitattributes parsing code

git may segfault if gitattributes contains an invalid
entry. A test is added to t0020 that triggers the segfault.
The parsing code is fixed to avoid the crash.

Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

attr.c

@@ -214,8 +214,11 @@ static struct match_attr *parse_attr_line(const char *line, const char *src,
 		num_attr = 0;
 		cp = name + namelen;
 		cp = cp + strspn(cp, blank);
-		while (*cp)
+		while (*cp) {
 			cp = parse_attr(src, lineno, cp, &num_attr, res);
+			if (!cp)
+				return NULL;
+		}
 		if (pass)
 			break;
 		res = xcalloc(1,

t/t0020-crlf.sh

@@ -371,4 +371,11 @@ test_expect_success 'in-tree .gitattributes (4)' '
 	}
 '
 
+test_expect_success 'invalid .gitattributes (must not crash)' '
+
+	echo "three +crlf" >>.gitattributes &&
+	git diff
+
+'
+
 test_done