Set prev to NULL so its garaunteed to have a value of some kind and

gcc doesn't explode.  Feel free to fix this correctly or whatever for
gcc builds.

This *should* quiesce tinderbox after r335347 for the gcc builds.

sys/contrib/libnv/nvpair.c

@@ -1979,7 +1979,7 @@ int
 nvpair_append_nvlist_array(nvpair_t *nvp, const nvlist_t *value)
 {
 	nvpair_t *tmpnvp;
-	nvlist_t *nvl, *prev;
+	nvlist_t *nvl, *prev = NULL;
 	int flags;
 
 	NVPAIR_ASSERT(nvp);