bpf: Check scalar or invalid register in check_helper_mem_access

Move the check for a NULL or zero register to check_helper_mem_access. This
makes check_stack_boundary easier to understand.

Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200921121227.255763-3-lmb@cloudflare.com