bpf: Fix NULL pointer dereference in __btf_resolve_helper_id()

bpf: Fix NULL pointer dereference in __btf_resolve_helper_id()

[ Upstream commit 5b801dfb7feb2738975d80223efc2fc193e55573 ]

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
Signed-off-by: Sasha Levin <sashal@kernel.org>