Sixteen: limit length of moves

The code that actually executes the moves can only cope with moves of
at most the width (or height as appropriate) of the grid.  Reject any
longer move, and for symmetry also negative moves of the same
magnitude.

Without this, the tile-moving code tends to access off the start of the
tile array.  To demonstrate this, build Sixteen with AddressSanitizer
and load this save file:

SAVEFILE:41:Simon Tatham's Portable Puzzle Collection
VERSION :1:1
GAME    :7:Sixteen
PARAMS  :3:4x4
CPARAMS :3:4x4
DESC    :38:2,16,3,10,13,8,7,4,9,14,12,11,15,1,5,6
NSTATES :1:2
STATEPOS:1:2
MOVE    :4:C1,9

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