ui/curses.c: Ensure we don't read off the end of curses2qemu array

Coverity spots that there is no bounds check before we
access the curses2qemu[] array.  Add one, bringing this
code path into line with the one that looks up entries
in curses2keysym[].

In theory getch() shouldn't return out of range keycodes,
but it's better not to assume this.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1470925407-23850-2-git-send-email-peter.maydell@linaro.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>