Fixed a few compilation warnings.

diff --git a/windows/multimon.c b/windows/multimon.c
index 1a4cfe4..0d0fef8 100644
--- a/windows/multimon.c
+++ b/windows/multimon.c
@@ -29,7 +29,7 @@
     {
         return xPRIMARY_MONITOR;
     }
-        return NULL;
+    return (HMONITOR)0;
 }
 
 /***********************************************************************
@@ -45,7 +45,7 @@
     {
         return xPRIMARY_MONITOR;
     }
-    return NULL;
+    return (HMONITOR)0;
 }
 
 /***********************************************************************
@@ -65,7 +65,7 @@
         return MonitorFromRect(&wp.rcNormalPosition, dwFlags);
     }
 
-    return NULL;
+    return (HMONITOR)0;
 }
 
 /***********************************************************************