gdb/python: don't allow the user to delete window title attributes

There's a bug in the python tui API.  If the user tries to delete the
window title attribute then this will trigger undefined behaviour in
GDB due to a missing nullptr check.

gdb/ChangeLog:

	* python/py-tui.c (gdbpy_tui_set_title): Check that the new value
	for the title is not nullptr.

gdb/testsuite/ChangeLog:

	* gdb.python/tui-window.exp: Add new tests.
	* gdb.python/tui-window.py (TestWindow) <__init__>: Store
	TestWindow object into global the_window.
	<remote_title>: New method.
	(delete_window_title): New function.

1 files changed, 9 insertions, 1 deletions

diff --git a/gdb/testsuite/gdb.python/tui-window.py b/gdb/testsuite/gdb.python/tui-window.py
index 88a1b06..3bea788 100644
--- a/gdb/testsuite/gdb.python/tui-window.py
+++ b/gdb/testsuite/gdb.python/tui-window.py
@@ -22,7 +22,7 @@ the_window = None
 class TestWindow:
     def __init__(self, win):
         global the_window
-        the_window = win
+        the_window = self
         self.count = 0
         self.win = win
         win.title = "This Is The Title"
@@ -34,8 +34,16 @@ class TestWindow:
         self.win.write("Test: " + str(self.count) + " " + str(w) + "x" + str(h))
         self.count = self.count + 1
 
+    # Tries to delete the title attribute.  GDB will throw an error.
+    def remove_title(self):
+        del self.win.title
+
 gdb.register_window_type("test", TestWindow)
 
+# Call REMOVE_TITLE on the global window object.
+def delete_window_title ():
+    the_window.remove_title ()
+
 # A TUI window "constructor" that always fails.
 def failwin(win):
     raise RuntimeError("Whoops")