realloc(x, 0) should not return NULL.

See http://www.open-std.org/jtc1/sc22/wg14/www/docs/summary.htm#dr_400.
Upstream jemalloc issue is opened by emaste at
https://github.com/jemalloc/jemalloc/issues/1629.

Reviewed by: emaste
PR: 240456
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
DIfferential revision: https://reviews.freebsd.org/D21632