lib/shadow/grp/: agetgroups(): Fix possible buffer overrun on non-Linux systems

lib/shadow/grp/: agetgroups(): Fix possible buffer overrun on non-Linux systems

Linux seems to at least write one group always from getgroups(2).
However, POSIX doesn't guarantee this, and a system might have 0 groups.

	It is implementation‐defined whether getgroups() also returns
	the effective group ID in the grouplist array.

Considering such a system, the call getgroups(0,NULL) could indeed
return 0, and the second call to getgroups might return a higher value,
if the group list has grown in between (race condition).  If this is the
case, we'd return an array of 0 elements (or 1, due to the MALLOC()
trick to avoid calling it with 0), with no elements filled, but where
ngids has been updated to have a positive value.  When the caller of
agetgroups() reads the array, they'd overrun the buffer.

Fixes: 05322ed89a1c (2025-01-24; "lib/shadow/grp/: agetgroups(): Add function")
Fixes: de941a7601f8 (2025-01-24; "lib/, src/: Simplify allocation of buffer")
Signed-off-by: Alejandro Colomar <alx@kernel.org>