Don't use alloca in addgetnetgrentX (BZ #16453)

Don't use alloca in addgetnetgrentX (BZ #16453)

addgetnetgrentX has a buffer which is grown as per the needs of the
requested size either by using alloca or by falling back to malloc if
the size is larger than 1K.  There are two problems with the alloca
bits: firstly, it doesn't really extend the buffer since it does not
use the return value of the extend_alloca macro, which is the location
of the reallocated buffer.  Due to this the buffer does not actually
extend itself and hence a subsequent write may overwrite stuff on the
stack.

The second problem is more subtle - the buffer growth on the stack is
discontinuous due to block scope local variables.  Combine that with
the fact that unlike realloc, extend_alloca does not copy over old
content and you have a situation where the buffer just has garbage in
the space where it should have had data.

This could have been fixed by adding code to copy over old data
whenever we call extend_alloca, but it seems unnecessarily
complicated.  This code is not exactly a performance hotspot (it's
called when there is a cache miss, so factors like network lookup or
file reads will dominate over memory allocation/reallocation), so this
premature optimization is unnecessary.

Thanks Brad Hubbard <bhubbard@redhat.com> for his help with debugging
the problem.