Make sure NC_HandleNCCalcSize returns a valid rectangle.
Painting fails if the rectangle is invalid.

windows/nonclient.c

diff --git a/windows/nonclient.c b/windows/nonclient.c
index 913a26b..942c0d3 100644
--- a/windows/nonclient.c
+++ b/windows/nonclient.c
@@ -566,6 +566,12 @@
 	    winRect->right  -= tmpRect.right;
 	    winRect->bottom -= tmpRect.bottom;
 	}
+
+        if (winRect->top > winRect->bottom)
+            winRect->bottom = winRect->top;
+
+        if (winRect->left > winRect->right)
+            winRect->right = winRect->left;
     }
     return result;
 }