netlink: Avoid passing NULL pointer to memcpy(), even with zero length.

In nl_msg_push_flag(), the 3rd NULL parameter causing 'memcpy()'
with NULL source pointer in nl_msg_push_unspec().

Found by Clang.

Signed-off-by: William Tu <u9012063@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>

lib/netlink.c

@@ -377,7 +377,7 @@ nl_msg_push_unspec(struct ofpbuf *msg, uint16_t type,
 void
 nl_msg_push_flag(struct ofpbuf *msg, uint16_t type)
 {
-    nl_msg_push_unspec(msg, type, NULL, 0);
+    nl_msg_push_unspec_uninit(msg, type, 0);
 }
 
 /* Prepends a Netlink attribute of the given 'type' and the given 8-bit 'value'