ws2_32/tests: Initialize a test buffer (valgrind)

[Bruno Jesus]

Fixes bug https://bugs.winehq.org/show_bug.cgi?id=36661
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diff --git a/dlls/ws2_32/tests/sock.c b/dlls/ws2_32/tests/sock.c
index 0bbce27..fb5ee1d 100644
--- a/dlls/ws2_32/tests/sock.c
+++ b/dlls/ws2_32/tests/sock.c
@@ -6833,6 +6833,7 @@ static void test_completion_port(void)
     GUID acceptExGuid = WSAID_ACCEPTEX;
     LPFN_ACCEPTEX pAcceptEx = NULL;
 
+    memset(buf, 0, sizeof(buf));
     previous_port = CreateIoCompletionPort(INVALID_HANDLE_VALUE, NULL, 0, 0);
     ok( previous_port != NULL, "Failed to create completion port %u\n", GetLastError());