bpf: fix segfault in test_verifier selftest

Minor fallout from merge resolution, test_verifier was segfaulting
because the REJECT result was correct, but errstr was NULL. Properly
fix it as in 339bbff2.

Fixes: 339bbff2 ("Merge git://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/testing/selftests/bpf/test_verifier.c

@@ -14194,7 +14194,8 @@ static struct bpf_test tests[] = {
 		},
 		.prog_type = BPF_PROG_TYPE_SOCKET_FILTER,
 		.errstr_unpriv = "function calls to other bpf functions are allowed for root only",
-                .result = REJECT,
+		.errstr = "!read_ok",
+		.result = REJECT,
 	},
 	{
 		"jset: functional",