selftests/bpf: fix RELEASE=1 compilation for sock_addr.c

When building selftests with RELEASE=1 using GCC compiler, it complaints
about uninitialized err. Fix the problem.

Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Martin KaFai Lau <[e-mail]>

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

···
 			break;
 		default:
 			ASSERT_TRUE(false, "Unknown sock addr test type");
+			err = -EINVAL;
 			break;
 		}