WP 34S and 31S bugs and fixes

[Bit]

With PRETTY_FRACTION_ENTRY enabled (as it is in the official build), longer fractions overflow into the exponent field because of the extra space and forward slash. The first such digit was displayed correctly but the second wasn't because disp_x() didn't take into account that there are fewer LCD segments in the exponent field.

Here's how to reproduce the problem:
[1] [2] [3] [4] [5] [.] [6] [7] [8] [9] [0] [.] [1] [2]

I've committed fixes for both the 34S and the 31S.