Fix NPE in getCompatibleMagnificationSpecForWindow

The commit 179a6b0 forgot to change the returned result.

Note that if the "spec" is non-null, the old code will
cause the source spec of DisplayMagnifier to be cumulatively
multiplied and then become infinite scale.

Bug: 180050756
Test: Enable compat scale and TalkBack
Change-Id: I5a647d4129165021054631461fb97aef61def390

services/core/java/com/android/server/wm/WindowManagerService.java

@@ -7531,8 +7531,8 @@ public class WindowManagerService extends IWindowManager.Stub
                 if (spec != null) {
                     result.setTo(spec);
                 }
-                spec.scale *= windowState.mGlobalScale;
-                return spec;
+                result.scale *= windowState.mGlobalScale;
+                return result;
             }
         }