sha1_loose_object_info: return error for corrupted objects

When sha1_loose_object_info() finds that a loose object file
cannot be stat(2)ed or mmap(2)ed, it returns -1 to signal an
error to the caller.  However, if it found that the loose
object file is corrupt and the object data cannot be used
from it, it stuffs OBJ_BAD into "type" field of the
object_info, but returns zero (i.e., success), which can
confuse callers.

This is due to 052fe5eac (sha1_loose_object_info: make type
lookup optional, 2013-07-12), which switched the return to a
strict success/error, rather than returning the type (but
botched the return).

Callers of regular sha1_object_info() don't notice the
difference, as that function returns the type (which is
OBJ_BAD in this case). However, direct callers of
sha1_object_info_extended() see the function return success,
but without setting any meaningful values in the object_info
struct, leading them to access potentially uninitialized
memory.

The easiest way to see the bug is via "cat-file -s", which
will happily ignore the corruption and report whatever
value happened to be in the "size" variable.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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