[readline] Fix double free in _rl_scxt_dispose

[readline] Fix double free in _rl_scxt_dispose

Consider the following scenario.  We start gdb in TUI mode:
...
$ gdb -q -tui
...
and type ^R which gives us the reverse-isearch prompt in the cmd window:
...
(reverse-i-search)`':
...
and then type "foo", right-arrow-key, and ^C.

In TUI mode, gdb uses a custom rl_getc_function tui_getc.

When pressing the right-arrow-key, tui_getc:
- attempts to scroll the TUI src window, without any effect, and
- returns 0.

The intention of returning 0 is mentioned here in tui_dispatch_ctrl_char:
...
  /* We intercepted the control character, so return 0 (which readline
     will interpret as a no-op).  */
  return 0;
...

However, after this 0 is returned by the rl_read_key () call in
_rl_search_getchar, _rl_read_mbstring is called, which incorrectly interprets
0 as the first part of an utf-8 multibyte char, and tries to read the next
char.

In this state, the ^C takes effect and we run into a double free because
_rl_isearch_cleanup is called twice.

Both these issues need fixing independently, though after fixing the first we
no longer trigger the second.

The first issue is caused by the subtle difference between:
- a char array containing 0 chars, which is zero-terminated, and
- a char array containing 1 char, which is zero.

In mbrtowc terms, this is the difference between:
...
  mbrtowc (&wc, "", 0, &ps);
...
which returns -2, and:
...
  mbrtowc (&wc, "", 1, &ps);
...
which returns 0.

Note that _rl_read_mbstring calls _rl_get_char_len without passing it an
explicit length parameter, and consequently it cannot distinguish between the
two, and defaults to the "0 chars" choice.

Note that the same problem doesn't exist in _rl_read_mbchar.

Fix this by defaulting to the "1 char" choice in _rl_get_char_len:
...
-  if (_rl_utf8locale && l > 0 && UTF8_SINGLEBYTE(*src))
+  if (_rl_utf8locale && l >= 0 && UTF8_SINGLEBYTE(*src))
...

The second problem happens when the call to _rl_search_getchar in
_rl_isearch_callback returns.  At that point _rl_isearch_cleanup has already
been called from the signal handler, but we proceed regardless, using a cxt
pointer that has been freed.

Fix this by checking for "RL_ISSTATE (RL_STATE_ISEARCH)" after the call to
_rl_search_getchar:
...
   c = _rl_search_getchar (cxt);
+  if (!RL_ISSTATE (RL_STATE_ISEARCH))
+    return 1;
...

Tested on x86_64-linux.

Approved-By: Chet Ramey <chet.ramey@case.edu>
PR tui/30056
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30056