- Having a fixed response for malloc(0) eliminates a source of
  implementation-defined behaviour.

- Returning NULL in response to a request for zero bytes was already
  allowed by the standard, although it is (to quote the FreeBSD man
  page) "a silly response to a silly question".

- Explicitly returning NULL in this case makes it easier for static
  analysis tools to recognize that NULL is possible and may help them to
  find more bugs.

If n is SIZE_MAX, then n+1 (used in the following IMALLOC) will overflow.

Reported by:	gcc -fanalyzer

This commit does not modify external/queue/queue.h.  That file has
1 instance of "while(" and 38 instances of "while (", but keeping it in
sync with the NetBSD source tree is more important than stylistic
consistency.