getline(3): Fix segfault caused by NULL pointer

getline(3): Fix segfault caused by NULL pointer

This fixes a segfault seen with pkg's audit function, and effectively syncs
getdelim with FreeBSD.

Taken-from: FreeBSD SVN 197752 (04 OCT 2009)

- Tolerate applications that pass a NULL pointer for the buffer and
  claim that the capacity of the buffer is nonzero.

- If an application passes in a non-NULL buffer pointer and claims the
  buffer has zero capacity, we should free (well, realloc) it anyway.
  It could have been obtained from malloc(0), so failing to free it
  would be a small memory leak.