unpack_sha1_header(): detect malformed object header

unpack_sha1_header(): detect malformed object header

When opening a loose object file, we often do this sequence:

 - prepare a short buffer for the object header (on stack)

 - call unpack_sha1_header() and have early part of the object data
   inflated, enough to fill the buffer

 - parse that data in the short buffer, assuming that the first part
   of the object is <typename> SP <length> NUL

Because the parsing function parse_sha1_header_extended() is not
given the number of bytes inflated into the header buffer, it you
craft a file whose early part inflates a garbage sequence without SP
or NUL, and replace a loose object with it, it will end up reading
past the end of the inflated data.

To correct this, do the following four things:

 - rename unpack_sha1_header() to unpack_sha1_short_header() and
   have unpack_sha1_header_to_strbuf() keep calling that as its
   helper function.  This will detect and report zlib errors, but is
   not aware of the format of a loose object (as before).

 - introduce unpack_sha1_header() that calls the same helper
   function, and when zlib reports it inflated OK into the buffer,
   check if the inflated data has NUL.  This would ensure that
   parsing function will terminate within the buffer that holds the
   inflated header.

 - update unpack_sha1_header_to_strbuf() to check if the resulting
   buffer has NUL for the same effect.

 - update parse_sha1_header_extended() to make sure that its loop to
   find the SP that terminates the <typename> stops at NUL.

Essentially, this makes unpack_*() functions that are asked to
unpack a loose object header to be a bit more strict and detect an
input that cannot possibly be a valid object header, even before the
parsing function kicks in.

Reported-by: Gustavo Grieco <gustavo.grieco@imag.fr>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>