Correct a logic error in Unequal game desc validation.

A user points out that the error check that should have detected a
non-digit where a digit should have been was never firing, due to an
&& that should have been ||.

I don't think it was a harmful error - the subsequent check that the
number was in range, plus the skipping past only digits to find the
next part of the string, combine to arrange that not many kinds of
invalid game id could actually get through.

But it did have the small effect that a 0 could be elided without
triggering an error, e.g. the game ids

  4:0,0,0,0,0,0L,0,0,0R,0U,0,0L,0,0,,3,
  4:0,0,0,0,0,0L,0,0,0R,0U,0,0L,0,0,0,3,

would both be accepted, and would be decoded into the same game, even
though the former should have failed syntax validation. Now it does.

unequal.c

@@ -1214,7 +1214,7 @@ static game_state *load_game(const game_params *params, const char *desc,
             why = "Too much data to fill grid"; goto fail;
         }
 
-        if (*p < '0' && *p > '9') {
+        if (*p < '0' || *p > '9') {
             why = "Expecting number in game description"; goto fail;
         }
         n = atoi(p);