tools: libxl: do not overrun input buffer in libxl__parse_mac

tools: libxl: do not overrun input buffer in libxl__parse_mac

Valgrind reports:
==7971== Invalid read of size 1
==7971==    at 0x40877BE: libxl__parse_mac (libxl_internal.c:288)
==7971==    by 0x405C5F8: libxl__device_nic_from_xs_be (libxl.c:3405)
==7971==    by 0x4065542: libxl__append_nic_list_of_type (libxl.c:3484)
==7971==    by 0x4065542: libxl_device_nic_list (libxl.c:3504)
==7971==    by 0x406F561: libxl_retrieve_domain_configuration (libxl.c:6661)
==7971==    by 0x805671C: reload_domain_config (xl_cmdimpl.c:2037)
==7971==    by 0x8057F30: handle_domain_death (xl_cmdimpl.c:2116)
==7971==    by 0x8057F30: create_domain (xl_cmdimpl.c:2580)
==7971==    by 0x805B4B2: main_create (xl_cmdimpl.c:4652)
==7971==    by 0x804EAB2: main (xl.c:378)

This is because on the final iteration the tok += 3 skips over the terminating
NUL to the next byte, and then *tok reads it. Fix this by using endptr as the
iterator.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Don Slutz <dslutz@verizon.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>