lib/sgetgrent.c: fix null pointer dereference

lib/sgetgrent.c: fix null pointer dereference

If reallocation fails in function list, then reset the size to 0 again.
Without the reset, the next call assumes that `members` points to
a memory location with reserved space.

Also use size_t instead of int for size to prevent signed integer
overflows. The length of group lines is not limited.

Fixes 45c0003e53ab671c63dcd530fd9f3245d3b29e76 (4.14 release series)

Reviewed-by: Alejandro Colomar <alx@kernel.org>
Signed-off-by: Samanta Navarro <ferivoz@riseup.net>