bpftool: Check for NULL ptr of btf in codegen_asserts

bpf_object__btf() can return a NULL value.  If bpf_object__btf returns
null, do not progress through codegen_asserts(). This avoids a null ptr
dereference at the call btf__type_cnt() in the function find_type_for_map()

Signed-off-by: Michael Mullin <masmullin@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220523194917.igkgorco42537arb@jup