Fix risk of Win32 timer setup call getting ignored (#1311)

If a user configures the configASSERT macro to expand to nothing,
a call to the Win32 API SetWaitableTimer() doesn't get compiled.

This can happen if, for example, configASSERT(x) set defined as
assert(x) (from assert.h) which expands to nothing when NDEBUG is
set (common for "release" builds).
diff --git a/portable/MSVC-MingW/port.c b/portable/MSVC-MingW/port.c
index 74dd777..540c4a4 100644
--- a/portable/MSVC-MingW/port.c
+++ b/portable/MSVC-MingW/port.c
@@ -152,6 +152,7 @@
     TickType_t xWaitTimeBetweenTicks = portTICK_PERIOD_MS;
     HANDLE hTimer = NULL;
     LARGE_INTEGER liDueTime;
+    BOOL bSuccess;
 
     /* Set the timer resolution to the maximum possible. */
     if( timeGetDevCaps( &xTimeCaps, sizeof( xTimeCaps ) ) == MMSYSERR_NOERROR )
@@ -190,7 +191,8 @@
 
     /* Set the Waitable Timer. The timer is set to run periodically at every
     xWaitTimeBetweenTicks milliseconds. */
-    configASSERT( SetWaitableTimer( hTimer, &liDueTime, xWaitTimeBetweenTicks, NULL, NULL, 0 ) );
+    bSuccess = SetWaitableTimer( hTimer, &liDueTime, xWaitTimeBetweenTicks, NULL, NULL, 0 );
+    configASSERT( bSuccess );
 
     while( xPortRunning == pdTRUE )
     {