gamedate: no negative turns allowed.

feed the beast assert.

src/bindings.c

@@ -921,7 +921,7 @@ static int init_data(const char *filename, const char *catalog)
     if (l) {
         return l;
     }
-    if (turn < 0) {
+    if (turn <= 0) {
         turn = first_turn();
     }
     return 0;

src/calendar.c

@@ -27,8 +27,7 @@ const gamedate *get_gamedate(int turn, gamedate * gd)
     int t = turn - first_turn();
 
     assert(gd);
-    if (t < 0)
-        t = turn;
+    assert(t >= 0);
 
     gd->week = t % weeks_per_month;       /* 0 - weeks_per_month-1 */
     gd->month = (t / weeks_per_month + first_month) % months_per_year;    /* 0 - months_per_year-1 */

src/kernel/config.c

@@ -96,7 +96,7 @@ OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 struct settings global;
 
 bool lomem = false;
-int turn = -1;
+int turn = 0;
 
 const char *parameters[MAXPARAMS] = {
     "LOCALE",

src/laws.c

@@ -4183,6 +4183,7 @@ static void reset_game(void)
 
 void turn_begin(void)
 {
+    assert(turn >= 0);
     ++turn;
     reset_game();
 }

src/tests.c

@@ -193,7 +193,7 @@ void test_log_stderr(int flags) {
 
 static void test_reset(void) {
     int i;
-    turn = 0;
+    turn = 1;
     default_locale = 0;
 
     if (errno) {