sha1_file: guard against invalid loose subdirectory numbers

sha1_file: guard against invalid loose subdirectory numbers

Loose object subdirectories have hexadecimal names based on the first
byte of the hash of contained objects, thus their numerical
representation can range from 0 (0x00) to 255 (0xff).  Change the type
of the corresponding variable in for_each_file_in_obj_subdir() and
associated callback functions to unsigned int and add a range check.

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>