tests: fix missing init in bgp_mp_attr_test.c

turns out, bgp_mp_reach_parse really doesn't like getting garbage
attribute input.  In particular, attr->extra better be NULL or we
merrily go trample random places (like our stack).

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/tests/bgp_mp_attr_test.c b/tests/bgp_mp_attr_test.c
index f086740..177c1ad 100644
--- a/tests/bgp_mp_attr_test.c
+++ b/tests/bgp_mp_attr_test.c
@@ -436,8 +436,8 @@
 {
   int ret;
   int oldfailed = failed;
-  struct attr attr;
-  struct bgp_nlri nlri;
+  struct attr attr = { };
+  struct bgp_nlri nlri = { };
   struct bgp_attr_parser_args attr_args = {
     .peer = peer,
     .length = t->len,