sha1_loose_object_info: handle errors from unpack_sha1_rest

When a caller of sha1_object_info_extended() sets the "contentp" field in object_info, we call unpack_sha1_rest() but do not check whether it signaled an error. This causes two problems: 1. We pass back NULL to the caller via the contentp field, but the function returns "0" for success. A caller might reasonably expect after a successful return that it can access contentp without a NULL check and segfault. As it happens, this is impossible to trigger in the current code. There is exactly one caller which uses contentp, read_object(). And the only thing it does after a successful call is to return the content pointer to its caller, using NULL as a sentinel for errors. So in effect it converts the success code from sha1_object_info_extended() back into an error! But this is still worth addressing avoid problems for future users of "contentp". 2. Callers of unpack_sha1_rest() are expected to close the zlib stream themselves on error. Which means that we're leaking the stream. The problem in (1) comes from from c84a1f3ed4 (sha1_file: refactor read_object, 2017-06-21), which added the contentp field. Before that, we called unpack_sha1_rest() via unpack_sha1_file(), which directly used the NULL to signal an error. But note that the leak in (2) is actually older than that. The original unpack_sha1_file() directly returned the result of unpack_sha1_rest() to its caller, when it should have been closing the zlib stream itself on error. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>