Fix pagination crash when the TUI is active

The TUI currently crashes when the user types <return> in response to
a pagination prompt:

  $ gdb --tui ...
  *the TUI is now active*
  (gdb) set height 2
  (gdb) help
  List of classes of commands:

  Program received signal SIGSEGV, Segmentation fault.
  strlen () at ../sysdeps/x86_64/strlen.S:106
  106             movdqu  (%rax), %xmm12

  (top-gdb) bt
  #0  strlen () at ../sysdeps/x86_64/strlen.S:106
  #1  0x000000000086be5f in xstrdup (s=0x0) at ../src/libiberty/xstrdup.c:33
  #2  0x00000000005163f9 in tui_prep_terminal (notused1=1) at ../src/gdb/tui/tui-io.c:296
  #3  0x000000000077a7ee in _rl_callback_newline () at ../src/readline/callback.c:82
  #4  0x000000000077a853 in rl_callback_handler_install (prompt=0x0, linefunc=0x618b60 <command_line_handler>) at ../src/readline/callback.c:102
  #5  0x0000000000718a5c in gdb_readline_wrapper_cleanup (arg=0xfd14d0) at ../src/gdb/top.c:788
  #6  0x0000000000596d08 in do_my_cleanups (pmy_chain=0xcf0b38 <cleanup_chain>, old_chain=0x1043d10) at ../src/gdb/cleanups.c:155
  #7  0x0000000000596d75 in do_cleanups (old_chain=0x1043d10) at ../src/gdb/cleanups.c:177
  #8  0x0000000000718bd9 in gdb_readline_wrapper (prompt=0x7fffffffcfa0 "---Type <return> to continue, or q <return> to quit---")
      at ../src/gdb/top.c:835
  #9  0x000000000071cf74 in prompt_for_continue () at ../src/gdb/utils.c:1894
  #10 0x000000000071d434 in fputs_maybe_filtered (linebuffer=0x1043db0 "List of classes of commands:\n\n", stream=0xf72e20, filter=1)
      at ../src/gdb/utils.c:2111
  #11 0x000000000071da0f in vfprintf_maybe_filtered (stream=0xf72e20, format=0x89aef8 "List of classes of %scommands:\n\n", args=0x7fffffffd118, filter=1)
      at ../src/gdb/utils.c:2339
  #12 0x000000000071da4a in vfprintf_filtered (stream=0xf72e20, format=0x89aef8 "List of classes of %scommands:\n\n", args=0x7fffffffd118)
      at ../src/gdb/utils.c:2347
  #13 0x000000000071dc72 in fprintf_filtered (stream=0xf72e20, format=0x89aef8 "List of classes of %scommands:\n\n") at ../src/gdb/utils.c:2399
  #14 0x00000000004f90ab in help_list (list=0xe6d100, cmdtype=0x89ad8c "", class=all_classes, stream=0xf72e20)
      at ../src/gdb/cli/cli-decode.c:1038
  #15 0x00000000004f8dba in help_cmd (arg=0x0, stream=0xf72e20) at ../src/gdb/cli/cli-decode.c:946

Git 0017922 added:

    @@ -776,6 +777,12 @@ gdb_readline_wrapper_cleanup (void *arg)

     gdb_assert (input_handler == gdb_readline_wrapper_line);
     input_handler = cleanup->handler_orig;
  +
  +  /* Reinstall INPUT_HANDLER in readline, without displaying a
  +     prompt.  */
  +  if (async_command_editing_p)
  +    rl_callback_handler_install (NULL, input_handler);

and tui_prep_terminal simply misses handling the case of a NULL
rl_prompt.

I also checked that readline's sources do similar checks.

gdb/
2014-07-24  Pedro Alves  <palves@redhat.com>

	* tui/tui-io.c (tui_prep_terminal): Handle NULL rl_prompt.
Automatic date update in version.in