vim-patch:8.0.0948: crash if timer closes window while dragging status line

Problem:    Crash if timer closes window while dragging status line.
Solution:   Check if the window still exists. (Yasuhiro Matsumoto, closes
            vim/vim#1979)
989a70c590

src/nvim/eval.c

@@ -9493,6 +9493,9 @@ static void f_getchar(typval_T *argvars, typval_T *rettv, FunPtr fptr)
         /* Find the window at the mouse coordinates and compute the
          * text position. */
         win = mouse_find_win(&row, &col);
+        if (win == NULL) {
+          return;
+        }
         (void)mouse_comp_pos(win, &row, &col, &lnum);
         for (wp = firstwin; wp != win; wp = wp->w_next)
           ++winnr;