Merge branch 'rs/checkout-am-fix-unborn' into maint

A few codepaths in "checkout" and "am" working on an unborn branch
tried to access an uninitialized piece of memory.

* rs/checkout-am-fix-unborn:
  am: check return value of resolve_refdup before using hash
  checkout: check return value of resolve_refdup before using hash

Author: gitster

builtin/am.c

@@ -2148,7 +2148,7 @@ static void am_abort(struct am_state *state)
 	am_rerere_clear();
 
 	curr_branch = resolve_refdup("HEAD", 0, curr_head.hash, NULL);
-	has_curr_head = !is_null_oid(&curr_head);
+	has_curr_head = curr_branch && !is_null_oid(&curr_head);
 	if (!has_curr_head)
 		hashcpy(curr_head.hash, EMPTY_TREE_SHA1_BIN);
 

builtin/checkout.c

@@ -835,7 +835,8 @@ static int switch_branches(const struct checkout_opts *opts,
 	int flag, writeout_error = 0;
 	memset(&old, 0, sizeof(old));
 	old.path = path_to_free = resolve_refdup("HEAD", 0, rev.hash, &flag);
-	old.commit = lookup_commit_reference_gently(rev.hash, 1);
+	if (old.path)
+		old.commit = lookup_commit_reference_gently(rev.hash, 1);
 	if (!(flag & REF_ISSYMREF))
 		old.path = NULL;