Forbid lines off the grid in Pearl

While they couldn't be generated in normal play, execute_move() would
permit lines and marks across the edge of the grid that would then
generate assertion failures ("dsf_update_completion: Assertion
`INGRID(state, bx, by)' failed.").

I've added a check to execute_move() that after updating a square, the
square doesn't have any lines or marks that leave the grid.

This save file demonstrated the problem:

SAVEFILE:41:Simon Tatham's Portable Puzzle Collection
VERSZON :1:1
GAME    :5:Pearl
PARAMS  :5:5x6dt
CPARAMS :5:5x6dt
DESC    :6:eeeeee
NSTATES :1:2
STATEPOS:1:1
MOVE    :6:F1,4,2

0 files changed, 0 insertions, 0 deletions