Fixed exit code retrieval of spawned processes on Windows

Use GetExitCodeProcess() on Windows to retrieve the exit code of a process in
OS:excute()

platform/windows/os_windows.cpp

@@ -466,8 +466,10 @@ Error OS_Windows::execute(const String &p_path, const List<String> &p_arguments,
 	ERR_FAIL_COND_V(ret == 0, ERR_CANT_FORK);
 
 	if (p_blocking) {
-		DWORD ret2 = WaitForSingleObject(pi.pi.hProcess, INFINITE);
+		WaitForSingleObject(pi.pi.hProcess, INFINITE);
 		if (r_exitcode) {
+			DWORD ret2;
+			GetExitCodeProcess(pi.pi.hProcess, &ret2);
 			*r_exitcode = ret2;
 		}