libbpf: Fix uninitialized variable in btf_parse_type_sec

Fix obvious unitialized variable use that wasn't reported by compiler. libbpf
Makefile changes to catch such errors are added separately.

Fixes: 3289959b ("libbpf: Support BTF loading and raw data output in both endianness")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20200929220604.833631-1-andriin@fb.com