tests: multipath: Fix use of uninitialized wildcards.

The test is passing stack-allocated uninitialized wildcards to the
multipath execution:

 WARNING: MemorySanitizer: use-of-uninitialized-value
  0 0x873ece in mf_get_value lib/meta-flow.c:797:23
  1 0x8998ad in mf_write_subfield_flow lib/meta-flow.c:3365:5
  2 0x90dcbc in nxm_reg_load lib/nx-match.c:1851:5
  3 0x8aaf7d in multipath_execute lib/multipath.c:60:5
  4 0x5fbbe5 in test_multipath_main tests/test-multipath.c:68:13
  5 0x5fb4fb in ovstest_wrapper_test_multipath_main__ tests/test-multipath.c:137:1
  6 0x6e3204 in ovs_cmdl_run_command__ lib/command-line.c:247:17
  7 0x6e2828 in ovs_cmdl_run_command lib/command-line.c:278:5
  8 0x4c3d2a in main tests/ovstest.c:133:9
  9 0x7faa2e in __libc_start_call_main
 10 0x7faa2e in __libc_start_main@GLIBC_2.2.5
 11 0x432b44 in _start (tests/ovstest+0x432b44)

This doesn't affect the logic of the test it seems, but it's not a
good thing to do.

Fixes: f74e7df7450d ("ofproto-dpif: Always un-wildcard fields that are being set.")
Acked-by: Mike Pattrick <mkp@redhat.com>
Acked-by: Eelco Chaudron <echaudro@redhat.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

tests/test-multipath.c

@@ -63,6 +63,7 @@ test_multipath_main(int argc, char *argv[])
             struct flow flow;
 
             flow_random_hash_fields(&flow);
+            flow_wildcards_init_catchall(&wc);
 
             mp.max_link = n - 1;
             multipath_execute(&mp, &flow, &wc);