Assert that the back-end has provided a background colour

If we're going to refer to a specific colour, it seems appropriate to
insist that it exists.

midend.c

@@ -1312,6 +1312,7 @@ float *midend_colours(midend *me, int *ncolours)
     float *ret;
 
     ret = me->ourgame->colours(me->frontend, ncolours);
+    assert(*ncolours >= 1);
 
     {
         int i;