[PATCH] Fix TUI text centering

[Tom Tromey]

>>>>> "Tom" == Tom Tromey <tom@tromey.com> writes:

Tom> In a couple of spots, the TUI tries to center some text in the window.
Tom> Andrew noticed that the calculation is done strangely and the text
Tom> ends up somewhat to the left of center.

Tom> This patch fixes the problem.

Tom> Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31355

I'm checking this in.

Tom