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

tools/testing/selftests/bpf/prog_tests/core_reloc.c

@@ -392,7 +392,7 @@ static struct core_reloc_test_case test_cases[] = {
 		.input = STRUCT_TO_CHAR_PTR(core_reloc_existence___minimal) {
 			.a = 42,
 		},
-		.input_len = sizeof(struct core_reloc_existence),
+		.input_len = sizeof(struct core_reloc_existence___minimal),
 		.output = STRUCT_TO_CHAR_PTR(core_reloc_existence_output) {
 			.a_exists = 1,
 			.b_exists = 0,