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

cddl/contrib/opensolaris/lib/libdtrace/common/drti.c

@@ -110,7 +110,7 @@ dtrace_dof_init(void)
 	Elf32_Ehdr *elf;
 #endif
 	dof_helper_t dh;
-	Link_map *lmp;
+	Link_map *lmp = NULL;
 #if defined(sun)
 	Lmid_t lmid;
 #else