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>

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