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

tools/bpf/bpftool/gen.c

@@ -474,6 +474,9 @@ static void codegen_asserts(struct bpf_object *obj, const char *obj_name)
 	const struct btf_type *sec;
 	char map_ident[256], var_ident[256];
 
+	if (!btf)
+		return;
+
 	codegen("\
 		\n\
 		__attribute__((unused)) static void			    \n\