Prevent a segfault when deleting a fullscreen frame on NextStep.

* nsterm.m ([EmacsView resetCursorRects:]): Be defensive when
accessing FRAME_OUTPUT_DATA.  [resetCursorRects:] can be called
from the event loop after the frame is deleted.  When this
happens, emacsframe is NULL.  This means there is an underlying
leak of the EmacsView object!  (Bug#59794)
Do not merge to master.

Copyright-paperwork-exempt: yes

src/nsterm.m

@@ -6703,8 +6703,16 @@ - (BOOL)acceptsFirstResponder
 
 - (void)resetCursorRects
 {
-  NSRect visible = [self visibleRect];
-  NSCursor *currentCursor = FRAME_POINTER_TYPE (emacsframe);
+  NSRect visible;
+  NSCursor *currentCursor;
+
+  /* On macOS 13, [resetCursorRects:] could be called even after the
+     window is closed. */
+  if (! emacsframe || ! FRAME_OUTPUT_DATA (emacsframe))
+    return;
+
+  visible = [self visibleRect];
+  currentCursor = FRAME_POINTER_TYPE (emacsframe);
   NSTRACE ("[EmacsView resetCursorRects]");
 
   if (currentCursor == nil)