patch 8.2.3759: quickfix buffer becomes hidden while still in a window

Problem:    Quickfix buffer becomes hidden while still in a window.
Solution:   Check if the closed window is the last window showing the quickfix
            buffer. (Yegappan Lakshmanan, closes #9303, closes #9300)
diff --git a/src/quickfix.c b/src/quickfix.c
index d7c0a88..1e25edb 100644
--- a/src/quickfix.c
+++ b/src/quickfix.c
@@ -2818,7 +2818,7 @@
 }
 
 /*
- * Find a window displaying a Vim help file.
+ * Find a window displaying a Vim help file in the current tab page.
  */
     static win_T *
 qf_find_help_win(void)
@@ -2893,8 +2893,8 @@
 }
 
 /*
- * Find a non-quickfix window in the current tabpage using the given location
- * list stack.
+ * Find a non-quickfix window using the given location list stack in the
+ * current tabpage.
  * Returns NULL if a matching window is not found.
  */
     static win_T *
@@ -2910,7 +2910,7 @@
 }
 
 /*
- * Find a window containing a normal buffer
+ * Find a window containing a normal buffer in the current tab page.
  */
     static win_T *
 qf_find_win_with_normal_buf(void)
@@ -2981,7 +2981,7 @@
 
     if (win == NULL)
     {
-	// Find the window showing the selected file
+	// Find the window showing the selected file in the current tab page.
 	FOR_ALL_WINDOWS(win)
 	    if (win->w_buffer->b_fnum == qf_fnum)
 		break;
@@ -4394,8 +4394,8 @@
 }
 
 /*
- * Find a window displaying the quickfix/location stack 'qi'
- * Only searches in the current tabpage.
+ * Find a window displaying the quickfix/location stack 'qi' in the current tab
+ * page.
  */
     static win_T *
 qf_find_win(qf_info_T *qi)
@@ -4410,7 +4410,7 @@
 
 /*
  * Find a quickfix buffer.
- * Searches in windows opened in all the tabs.
+ * Searches in windows opened in all the tab pages.
  */
     static buf_T *
 qf_find_buf(qf_info_T *qi)