Don't allow moves that change the constraints in Unequal

Unequal has a flags word per cell.  Some of those flags are fixed,
like the locations of the ">" signs, but others indicate errors and
are used to allow the player to mark clues as "spent".  Move strings
beginning with "F" allow the user to change the "spent" flags, but
they shouldn't allow the user to change any other flags, especially
those marking the constraints.

Without this fix, the following save file gives a "solver_nminmax:
Assertion `x >= 0 && y >= 0 && x < o && y < o' failed" after it adds a
clue that points off the board:

SAVEFILE:41:Simon Tatham's Portable Puzzle Collection
GAME    :7:Unequal
PARAMS  :3:3e0
CPARAMS :3:3e0
DESC    :17:0,0,0,0,0,0,0,0,0
NSTATES :2:3
STATEPOS:1:3
MOVE    :6:F2,0,4
MOVE    :1:H

0 files changed, 0 insertions, 0 deletions