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

[will]

Summary

See $SUBJECT. 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.

Test Plan

I wish I could remember how I hit this issue. This change was made locally
nearly three months ago.

However, in general it's a good idea to initialize variables that may not
be set by a call.

[markj]

Thanks!

[will]

Closed by commit rS275845 (authored by @will).