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/edit.c

@@ -7884,7 +7884,11 @@ static void ins_mousescroll(int dir)
     col = mouse_col;
 
     /* find the window at the pointer coordinates */
-    curwin = mouse_find_win(&row, &col);
+    win_T *const wp = mouse_find_win(&row, &col);
+    if (wp == NULL) {
+      return;
+    }
+    curwin = wp;
     curbuf = curwin->w_buffer;
   }
   if (curwin == old_curwin)

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;

src/nvim/mouse.c

@@ -125,6 +125,9 @@ retnomove:
 
     // find the window where the row is in
     wp = mouse_find_win(&row, &col);
+    if (wp == NULL) {
+      return IN_UNKNOWN;
+    }
     dragwin = NULL;
     // winpos and height may change in win_enter()!
     if (row >= wp->w_height) {                  // In (or below) status line
@@ -427,6 +430,7 @@ bool mouse_comp_pos(win_T *win, int *rowp, int *colp, linenr_T *lnump)
 
 // Find the window at screen position "*rowp" and "*colp".  The positions are
 // updated to become relative to the top-left of the window.
+// Returns NULL when something is wrong.
 win_T *mouse_find_win(int *rowp, int *colp)
 {
   frame_T     *fp;
@@ -450,7 +454,14 @@ win_T *mouse_find_win(int *rowp, int *colp)
       }
     }
   }
-  return fp->fr_win;
+  // When using a timer that closes a window the window might not actually
+  // exist.
+  FOR_ALL_WINDOWS_IN_TAB(wp, curtab) {
+    if (wp == fp->fr_win) {
+      return wp;
+    }
+  }
+  return NULL;
 }
 
 /*

src/nvim/normal.c

@@ -3995,7 +3995,11 @@ static void nv_mousescroll(cmdarg_T *cap)
     col = mouse_col;
 
     /* find the window at the pointer coordinates */
-    curwin = mouse_find_win(&row, &col);
+    win_T *const wp = mouse_find_win(&row, &col);
+    if (wp == NULL) {
+      return;
+    }
+    curwin = wp;
     curbuf = curwin->w_buffer;
   }