gh-144759: Fix undefined behavior from NULL pointer arithmetic in lexer (#144788)

Guard against NULL pointer arithmetic in `_PyLexer_remember_fstring_buffers`
and `_PyLexer_restore_fstring_buffers`. When `start` or `multi_line_start`
are NULL (uninitialized in tok_mode_stack[0]), performing `NULL - tok->buf`
is undefined behavior. Add explicit NULL checks to store -1 as sentinel
and restore NULL accordingly.

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.