gdb/tui: fix 'tui reg next/prev' command when data window is hidden

gdb/tui: fix 'tui reg next/prev' command when data window is hidden

Start GDB like:

  $ gdb -q executable
  (gdb) start
  (gdb) layout src
  ... tui windows are now displayed ...
  (gdb) tui reg next

At this point the data (register) window should be displayed, but will
contain the message 'Register Values Unavailable', and at the console
you'll see the message "unknown register group 'next'".

The same happens with 'tui reg prev' (but the error message is
slightly different).

At this point you can continue to use 'tui reg next' and/or 'tui reg
prev' and you'll keep getting the error message.

The problem is that when the data (register) window is first
displayed, it's current register group is nullptr.  As a consequence
tui_reg_next and tui_reg_prev (tui/tui-regs.c) will always just return
nullptr, which triggers an error in tui_reg_command.

In this commit I change tui_reg_next and tui_reg_prev so that they
instead return the first and last register group respectively if the
current register group is nullptr.

So, after this, using 'tui reg next' will (in the above case) show the
first register group, while 'tui reg prev' will display the last
register group.