gh-144759: Add regression test for NULL pointer arithmetic in lexer

Add test_lexer_buffer_realloc_with_null_start to test_repl.py that
exercises the code path where the lexer buffer is reallocated while
tok_mode_stack[0] has NULL start/multi_line_start pointers. This
triggers _PyLexer_remember_fstring_buffers and verifies the NULL
checks prevent undefined behavior.

Author: raminfp

Lib/test/test_repl.py

@@ -143,6 +143,22 @@ def test_multiline_string_parsing(self):
         output = kill_python(p)
         self.assertEqual(p.returncode, 0)
 
+    @cpython_only
+    def test_lexer_buffer_realloc_with_null_start(self):
+        # gh-144759: NULL pointer arithmetic in the lexer when start and
+        # multi_line_start are NULL (uninitialized in tok_mode_stack[0])
+        # and the lexer buffer is reallocated while parsing long input.
+        long_value = "a" * 2000
+        user_input = dedent(f"""\
+        x = f'{{{long_value!r}}}'
+        print(x)
+        """)
+        p = spawn_repl()
+        p.stdin.write(user_input)
+        output = kill_python(p)
+        self.assertEqual(p.returncode, 0)
+        self.assertIn(long_value, output)
+
     def test_close_stdin(self):
         user_input = dedent('''
             import os