bpftool: Fix maybe-uninitialized warnings

Somehow when bpftool is compiled in -Og mode, compiler produces new warnings
about possibly uninitialized variables. Fix all the reported problems.

Fixes: 2119f218 ("bpftool: add C output format option to btf dump subcommand")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210313210920.1959628-3-andrii@kernel.org