Initialize an argument to NULL instead of expecting dlinfo() to do it.

dlinfo() is a weak reference that may not be initialized at the time of
execution. The default implementation (in lib/libc/gen/dlfcn.c) neither
modifies the address pointed to by the third argument nor returns an error.

Differential Revision:	https://reviews.freebsd.org/D1326
Reviewed by:		markj
MFC after:		1 week