bpftool: Fix error return value in build_btf_type_table

An appropriate return value should be set on the failed path.

Fixes: 4d374ba0 ("tools: bpftool: implement "bpftool btf show|list"")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20201124104100.491-1-thunder.leizhen@huawei.com