setupapi/tests: Avoid side-effect in an assert.

dlls/setupapi/tests/install.c

@@ -63,10 +63,12 @@ static BOOL (WINAPI *pSetupGetInfFileListW)(PCWSTR, DWORD, PWSTR, DWORD, PDWORD)
 static void create_inf_file(LPCSTR filename, const char *data)
 {
     DWORD res;
+    BOOL ret;
     HANDLE handle = CreateFileA(filename, GENERIC_WRITE, 0, NULL,
                            CREATE_ALWAYS, FILE_ATTRIBUTE_NORMAL, NULL);
     assert(handle != INVALID_HANDLE_VALUE);
-    assert(WriteFile(handle, data, strlen(data), &res, NULL));
+    ret = WriteFile(handle, data, strlen(data), &res, NULL);
+    assert(ret != 0);
     CloseHandle(handle);
 }