for-each-ref: cope with tags with incomplete lines

If you have a tag with a single, incomplete line as its payload, asking
git-for-each-ref for its %(body) element accessed a NULL pointer.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t6300-for-each-ref.sh b/t/t6300-for-each-ref.sh
index a3c8941..8ced593 100755
--- a/t/t6300-for-each-ref.sh
+++ b/t/t6300-for-each-ref.sh
@@ -262,4 +262,14 @@
 	"
 done
 
+test_expect_success 'an unusual tag with an incomplete line' '
+
+	git tag -m "bogo" bogo &&
+	bogo=$(git cat-file tag bogo) &&
+	bogo=$(printf "%s" "$bogo" | git mktag) &&
+	git tag -f bogo "$bogo" &&
+	git for-each-ref --format "%(body)" refs/tags/bogo
+
+'
+
 test_done