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>

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