bpf: Fix NULL pointer dereference in __btf_resolve_helper_id()

Prevent __btf_resolve_helper_id() from dereferencing `btf_vmlinux`
as NULL. This patch fixes the following syzbot bug:

    https://syzkaller.appspot.com/bug?id=f823224ada908fa5c207902a5a62065e53ca0fcc

Reported-by:  <syzbot+ee09bda7017345f1fbe6@syzkaller.appspotmail.com>
Signed-off-by: Peilin Ye <yepeilin.cs@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200714180904.277512-1-yepeilin.cs@gmail.com