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
            #1979)
diff --git a/src/edit.c b/src/edit.c
index e083708..3a9b3aa 100644
--- a/src/edit.c
+++ b/src/edit.c
@@ -9418,7 +9418,7 @@
 {
     pos_T	tpos;
 # if defined(FEAT_WINDOWS)
-    win_T	*old_curwin = curwin;
+    win_T	*old_curwin = curwin, *wp;
 # endif
 # ifdef FEAT_INS_EXPAND
     int		did_scroll = FALSE;
@@ -9435,7 +9435,10 @@
 	col = mouse_col;
 
 	/* find the window at the pointer coordinates */
-	curwin = mouse_find_win(&row, &col);
+	wp = mouse_find_win(&row, &col);
+	if (wp == NULL)
+	    return;
+	curwin = wp;
 	curbuf = curwin->w_buffer;
     }
     if (curwin == old_curwin)