[gdb/tui] Fix resizing of terminal to 1 or 2 lines

When starting TUI in a terminal with 3 lines:
...
$ echo $LINES
3
$ gdb -q -tui
...
and resizing the terminal to 2 lines we run into a segfault.

The problem is that for the source window:
- the minimum height is 3 (the default), but
- the maximum height is only 2 because there are only 2 lines.

This discrepancy eventually leads to a call to newwin in make_window with:
...
(gdb) p height
$1 = 3
(gdb) p width
$2 = 56
(gdb) p y
$3 = -1
(gdb) p x
$4 = 0
...
which results in a nullptr.

This violates the assumption here in tui_apply_current_layout:
....
  /* Get the new list of currently visible windows.  */
  std::vector<tui_win_info *> new_tui_windows;
  applied_layout->get_windows (&new_tui_windows);
...
that get_windows only returns visible windows, which leads to tui_windows
holding a dangling pointer, which results in the segfault.

Fix this by:
- making sure get_windows only returns visible windows, and
- detecting the situation and dropping windows from the layout if
  there's no room for them.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>

PR tui/31044
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31044

gdb/testsuite/gdb.tui/resize.exp

@@ -39,3 +39,12 @@ Term::check_contents "source at startup" "\\|.*21 *return 0"
 
 Term::resize 40 90
 Term::check_box "source box after resize" 0 0 90 26
+
+# Check that resizing to less than 3 lines doesn't cause problems.
+foreach lines { 2 1 } {
+    with_test_prefix lines=$lines {
+	Term::resize $lines 90 0
+	Term::wait_for ""
+	Term::check_region_contents "has prompt" 0 0 90 $lines "$gdb_prompt"
+    }
+}

gdb/tui/tui-layout.c

@@ -452,6 +452,13 @@ tui_layout_window::apply (int x_, int y_, int width_, int height_,
   width = width_;
   height = height_;
   gdb_assert (m_window != nullptr);
+  if (width == 0 || height == 0)
+    {
+      /* The window was dropped, so it's going to be deleted, reset the
+	 soon to be dangling pointer.  */
+      m_window = nullptr;
+      return;
+    }
   m_window->resize (height, width, x, y);
 }
 
@@ -833,6 +840,7 @@ tui_layout_split::apply (int x_, int y_, int width_, int height_,
   int available_size = m_vertical ? height : width;
   int last_index = -1;
   int total_weight = 0;
+  int prev = -1;
   for (int i = 0; i < m_splits.size (); ++i)
     {
       bool cmd_win_already_exists = TUI_CMD_WIN != nullptr;
@@ -864,6 +872,14 @@ tui_layout_split::apply (int x_, int y_, int width_, int height_,
 	  info[i].max_size = info[i].min_size;
 	}
 
+      if (info[i].min_size > info[i].max_size)
+	{
+	  /* There is not enough room for this window, drop it.  */
+	  info[i].min_size = 0;
+	  info[i].max_size = 0;
+	  continue;
+	}
+
       if (info[i].min_size == info[i].max_size)
 	available_size -= info[i].min_size;
       else
@@ -874,10 +890,12 @@ tui_layout_split::apply (int x_, int y_, int width_, int height_,
 
       /* Two adjacent boxed windows will share a border, making a bit
 	 more size available.  */
-      if (i > 0
+      if (prev != -1
-	  && m_splits[i - 1].layout->last_edge_has_border_p ()
+	  && m_splits[prev].layout->last_edge_has_border_p ()
 	  && m_splits[i].layout->first_edge_has_border_p ())
 	info[i].share_box = true;
+
+      prev = i;
     }
 
   /* If last_index is set then we have a window that is not of a fixed

gdb/tui/tui-layout.h

@@ -189,7 +189,11 @@ public:
   /* See tui_layout_base::get_windows.  */
   void get_windows (std::vector<tui_win_info *> *windows) override
   {
-    windows->push_back (m_window);
+    if (m_window != nullptr && m_window->is_visible ())
+      {
+	/* Only get visible windows.  */
+	windows->push_back (m_window);
+      }
   }
 
 protected: