patcham: check return value of resolve_refdup before using hash

> If resolve_refdup() fails it returns NULL and possibly leaves its hash
> output parameter untouched.  Make sure to use it only if the function
> succeeded, in order to avoid accessing uninitialized memory.
> 
> Signed-off-by: Rene Scharfe <l.s.r@web.de>
> ---
>  builtin/am.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/builtin/am.c b/builtin/am.c
> index a95dd8b4e6..2c52c820aa 100644
> --- a/builtin/am.c
> +++ b/builtin/am.c
> @@ -2156,7 +2156,7 @@ 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);