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>

sha1_file.c

@@ -2963,10 +2963,14 @@ static int sha1_loose_object_info(const unsigned char *sha1,
 	} else if ((status = parse_sha1_header_extended(hdr, oi, flags)) < 0)
 		status = error("unable to parse %s header", sha1_to_hex(sha1));
 
-	if (status >= 0 && oi->contentp)
+	if (status >= 0 && oi->contentp) {
 		*oi->contentp = unpack_sha1_rest(&stream, hdr,
 						 *oi->sizep, sha1);
-	else
+		if (!*oi->contentp) {
+			git_inflate_end(&stream);
+			status = -1;
+		}
+	} else
 		git_inflate_end(&stream);
 
 	munmap(map, mapsize);