ui/curses.c: Clean up nextchr logic

Coverity identifies that at the top of the while(1) loop
in curses_refresh() the variable nextchr is always ERR,
and so the else case of the first if() is dead code.
Remove this dead code, and narrow the scope of the
nextchr variable to the place where it's used.

(This confused logic has been present since the curses
code was added to QEMU in 2008.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1470925407-23850-3-git-send-email-peter.maydell@linaro.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
