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 <andrii@kernel.org>
Link: https://lore.kernel.org/r/20240730231805.1933923-1-andrii@kernel.org
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

Author: anakryiko

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

@@ -2642,6 +2642,7 @@ void test_sock_addr(void)
 			break;
 		default:
 			ASSERT_TRUE(false, "Unknown sock addr test type");
+			err = -EINVAL;
 			break;
 		}