Fix null pointer when drawable of a GC is null

The drawable on a GC can be null this would result in a null pointer
error in GC#calculateZoomForImage as we check if the drawable is not
autoScalable.

Author: arunjose696

bundles/org.eclipse.swt/Eclipse SWT/win32/org/eclipse/swt/graphics/GC.java

@@ -1148,7 +1148,7 @@ private int calculateZoomForImage(int gcZoom, int srcWidth, int srcHeight, int d
 			// unscaled images can use the GC zoom
 			return gcZoom;
 		}
-		if (!drawable.isAutoScalable()) {
+		if (drawable != null && !drawable.isAutoScalable()) {
 			return gcZoom;
 		}