Mines: Don't check if the player has won if they've already lost

It can't happen in normal play, but if a save file had a "C" (clear
around) move that set off a mine, Mines could end up hitting an
assertion failure, "ncovered >= nmines".  This was because it would
check if the player had won by counting covered squares and mines, and
of course an uncovered mine is no longer a covered square but is still a
mine.

Since winning after you're dead isn't possible (at least in Mines), we
now skip the check entirely if the player has already died.

This save file demonstrates the problem:

SAVEFILE:41:Simon Tatham's Portable Puzzle Collection
GAME    :5:Mines
PARAMS  :1:7
CPARAMS :1:7
DESC    :22:r31,u,0000C000d0000020
NSTATES :1:2
STATEPOS:1:1
MOVE    :4:C6,2

mines.c

@@ -2146,6 +2146,8 @@ static int open_square(game_state *state, int x, int y)
 	    break;
     }
 
+    /* If the player has already lost, don't let them win as well. */
+    if (state->dead) return 0;
     /*
      * Finally, scan the grid and see if exactly as many squares
      * are still covered as there are mines. If so, set the `won'