Tolerate incorrect solutions in Inertia

The "solve" operation in Inertia generates a proposed solution as a
move string.  But if such a move string is loaded from a save file it
might not actually describe a solution.  If that happens then it's
possible to reach the end of the "solution" without winning, and doing
so should probably cause a recalculation of the solution rather than
an assertion failure ("execute_move: Assertion `ret->solnpos <
ret->soln->len' failed.").

I am a little concerned by the way that normal solve operations end up
encoded in the save file, but the re-solvings caused by going off
course don't, but I haven't got a good answer to that.

Here's a save file that demonstrates the assertion failure:

SAVEFILE:41:Simon Tatham's Portable Puzzle Collection
GAME    :7:Inertia
PARAMS  :3:8x8
CPARAMS :3:8x8
DESC    :64:sbgwsmswwgggwggmmbwgwbssbwbsbwbbwsSmwbbsbbmggbmssgmgwbmmwmbmmwsw
NSTATES :2:3
STATEPOS:1:1
MOVE 000:2:S0
MOVE 000:2:00

0 files changed, 0 insertions, 0 deletions