selftests/bpf: Fix invalid memory reads in core_relo selftest

Another one found by AddressSanitizer. input_len is bigger than actually
initialized data size.

Fixes: c7566a69 ("selftests/bpf: Add field existence CO-RE relocs tests")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200429012111.277390-8-andriin@fb.com