getgrent_next_nss (compat-initgroups): Remove alloca fallback [BZ #18023]

If the caller-supplied buffer is not large enough, fall back directly
malloc.

The previous __libc_use_alloca check was incorrect because it did not
take into account that extend_alloca may fail to merge allocations, so
it would underestimate the stack space being used by roughly a factor
of two.