builtin/checkout: handle -B from detached HEAD correctly

Ensure that strcmp() isn't called when head is null.

Previously we were getting segfaults when checkout -B was done from a
detached HEAD.

Signed-off-by: Tay Ray Chuan <rctay89@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

branch.c

@@ -159,7 +159,7 @@ void create_branch(const char *head,
 			dont_change_ref = 1;
 		else if (!force)
 			die("A branch named '%s' already exists.", name);
-		else if (!is_bare_repository() && !strcmp(head, name))
+		else if (!is_bare_repository() && head && !strcmp(head, name))
 			die("Cannot force update the current branch.");
 		forcing = 1;
 	}

t/t2018-checkout-branch.sh

@@ -124,6 +124,12 @@ test_expect_success 'checkout -B to an existing branch resets branch to HEAD' '
 	do_checkout branch2 "" -B
 '
 
+test_expect_success 'checkout -B to an existing branch from detached HEAD resets branch to HEAD' '
+	git checkout $(git rev-parse --verify HEAD) &&
+
+	do_checkout branch2 "" -B
+'
+
 test_expect_success 'checkout -B to an existing branch with an explicit ref resets branch to that ref' '
 	git checkout branch1 &&