vim-patch:8.2.4248: no proper test for moving the window separator

Problem:    No proper test for moving the window separator.
Solution:   Add a test.  Add comment in code. (closes vim/vim#9656)
a0c4e2f2d7

Remove the assertion as it is now possible for `fr` to be `NULL`.

The test fails without clearing messages. Not sure if this is a bug.

src/nvim/window.c

@@ -5795,7 +5795,6 @@ void win_drag_vsep_line(win_T *dragwin, int offset)
     }
     fr = curfr;  // put fr at window that grows
   }
-  assert(fr);
 
   // Not enough room
   if (room < offset) {
@@ -5808,7 +5807,9 @@ void win_drag_vsep_line(win_T *dragwin, int offset)
   }
 
   if (fr == NULL) {
-    return;  // Safety check, should not happen.
+    // This can happen when calling win_move_separator() on the rightmost
+    // window.  Just don't do anything.
+    return;
   }
 
   // grow frame fr by offset lines