libbpf: Fix signedness bug in btf_dump_array_data()

The btf__resolve_size() function returns negative error codes so
"elem_size" must be signed for the error handling to work.

Fixes: 920d16af ("libbpf: BTF dumper support for typed data")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20220208071552.GB10495@kili