patch 8.0.1496: clearing a pointer takes two lines

Problem:    Clearing a pointer takes two lines.
Solution:   Add VIM_CLEAR() and replace vim_clear(). (Hirohito Higashi,
            closes #2629)
diff --git a/src/os_win32.c b/src/os_win32.c
index a42df6f..121ff2a 100644
--- a/src/os_win32.c
+++ b/src/os_win32.c
@@ -2252,8 +2252,7 @@
 	cb->Regions = (PSMALL_RECT)alloc(cb->NumRegions * sizeof(SMALL_RECT));
 	if (cb->Regions == NULL)
 	{
-	    vim_free(cb->Buffer);
-	    cb->Buffer = NULL;
+	    VIM_CLEAR(cb->Buffer);
 	    return FALSE;
 	}
     }
@@ -2278,10 +2277,8 @@
 		BufferCoord,			/* offset in our buffer */
 		&ReadRegion))			/* region to save */
 	{
-	    vim_free(cb->Buffer);
-	    cb->Buffer = NULL;
-	    vim_free(cb->Regions);
-	    cb->Regions = NULL;
+	    VIM_CLEAR(cb->Buffer);
+	    VIM_CLEAR(cb->Regions);
 	    return FALSE;
 	}
 	cb->Regions[i] = ReadRegion;