gdb/tui: avoid theoretical bug with 'tui reg' command

gdb/tui: avoid theoretical bug with 'tui reg' command

While looking at the 'tui reg' command as part of another patch, I
spotted a theoretical bug.

The 'tui reg' command takes the name of a register group, but also
handles partial register group matches, though the partial match has to
be unique.  The current command logic goes:

With the code as currently written, if a target description named a
register group either 'prev' or 'next' then GDB would see this as an
ambiguous register name, and refuse to switch groups.

Naming a register group 'prev' or 'next' seems pretty unlikely, but,
by adding a single else block we can prevent this problem.

Now, if there's a 'prev' or 'next' register group, the user will not
be able to select the group directly, the 'prev' and 'next' names will
always iterate through the available groups instead.  But at least the
user could select their groups by iteration, rather than direct
selection.