Tighten grid-size limit in Mines

Mines uses random_upto() to decide where to place mines, and
random_upto() takes a maximum limit of 2^28-1, so limit the number of
grid squares to that (or INT_MAX if someone's still trying to build on
a 16-bit system).

This avoids an assertion failure: "random_upto: Assertion `bits < 32'
failed." which can be demonstrated by this save file:

SAVEFILE:41:Simon Tatham's Portable Puzzle Collection
VERSION :1:1
GAME    :5:Mines
PARAMS  :5:18090
CPARAMS :5:18090
DESC    :11:r9,u,MEdff6
UI      :2:D0
TIME    :1:0
NSTATES :1:2
STATEPOS:1:2
MOVE    :4:O2,1

0 files changed, 0 insertions, 0 deletions