unpack_sha1_file(): detect corrupt loose object files.

unpack_sha1_file(): detect corrupt loose object files.

We did not detect broken loose object files, either when
underlying inflate() signalled the breakage, nor inflate()
finished and we had garbage trailing at the end.  We do better
now.

We also make unpack_sha1_file() a static function to
sha1_file.c, since it is not used by anybody outside.

Signed-off-by: Junio C Hamano <junkio@cox.net>