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>