Avoid invalid moves when solving Tracks

The solver, when it decided that an edge or square should be both TRACK
and NOTRACK, would correctly decide that the puzzle was insoluble, but
would also mark the edge with both flags in its working copy.  This
could then lead to assertion violations if that working copy of the
board was used for something else, for instance if it was fed back into
the solver.  This couldn't happen in normal play, since failed solutions
just cause the solve command to fail, but the diagnostic "H" command
could trigger it from a save file, causing an assertion failure:
"state->sflags[y*state->p.w + x] & S_CLUE".

Now when the solver runs into this situation, it marks the puzzle as
insoluble but doesn't set the invalid flag, so the board remains valid
and future solve operations are safe.

This save file is the one that demonstrated the problem:

SAVEFILE:41:Simon Tatham's Portable Puzzle Collection
GAME    :12:Train Tracks
PARAMS  :5:6x6t0
CPARAMS :5:6x6t0
DESC    :31:b0t9l,,S0,00,0,0,4,0,0,S0,0,0,0
NSTATES :1:8
STATEPOS:1:2
MOVE    :1:H
MOVE    :1:H
MOVE    :1:H
MOVE    :1:H
MOVE    :1:H
MOVE    :1:H
MOVE    :1:H

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