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Make sure that moves in Flood use only valid colours

If execute_move() receieves a move that uses a colour beyond the range
for the current game, it now rejects it.  Without this a solve string
containing an invalid colour would cause an assertion failure: "fill:
Assertion `oldcolour != newcolour' failed."  While I was in the area I
put a range check on colours for normal moves as well.  To demonstrate
the problem, load this save file:

SAVEFILE:41:Simon Tatham's Portable Puzzle Collection
VERSION :1:1
GAME    :5:Flood
PARAMS  :7:6x6c6m5
CPARAMS :7:6x6c6m3
DESC    :39:432242034203340350204502505323231342,17
NSTATES :1:2
STATEPOS:1:2
MOVE    :2:S6

--- a/flood.c

+++ b/flood.c

@@ -886,7 +886,7 @@

     if (move[0] == 'M' &&

         sscanf(move+1, "%d", &c) == 1 &&

-        c >= 0 &&

+        c >= 0 && c < state->colours &&

         c != state->grid[FILLY * state->w + FILLX] &&

         !state->complete) {

         int *queue = snewn(state->w * state->h, int);

@@ -945,10 +945,12 @@

                 return NULL;

             };

             sol->moves[i] = atoi(p);

-            if (i == 0 ?

-                sol->moves[i] == state->grid[FILLY * state->w + FILLX] :

-                sol->moves[i] == sol->moves[i-1])

-                /* Solution contains a fill with the current colour. */

+            if (sol->moves[i] < 0 || sol->moves[i] >= state->colours ||

+                (i == 0 ?

+                 sol->moves[i] == state->grid[FILLY * state->w + FILLX] :

+                 sol->moves[i] == sol->moves[i-1]))

+                /* Solution contains a fill with an invalid colour or

+                 * the current colour. */

                 goto badsolve;

             p += strspn(p, "0123456789");

             if (*p) {