[bugfix] Respect rot_spawn_chance

I broke it in CleverRaven#48116,
made it treat everything as if it had rot_spawn_chance = 100. Was most
noticable for unfertilized eggs, which really shouldn't hatch.

src/map.cpp

@@ -6988,13 +6988,16 @@ void map::rotten_item_spawn( const item &item, const tripoint &pnt )
     if( mgroup.is_null() ) {
         return;
     }
-    MonsterGroupResult spawn_details = MonsterGroupManager::GetResultFromGroup( mgroup );
-    add_spawn( spawn_details, pnt );
-    if( get_player_view().sees( pnt ) ) {
-        if( item.is_seed() ) {
-            add_msg( m_warning, _( "Something has crawled out of the %s plants!" ), item.get_plant_name() );
-        } else {
-            add_msg( m_warning, _( "Something has crawled out of the %s!" ), item.tname() );
+
+    if( rng( 0, 100 ) < comest->rot_spawn_chance ) {
+        MonsterGroupResult spawn_details = MonsterGroupManager::GetResultFromGroup( mgroup );
+        add_spawn( spawn_details, pnt );
+        if( get_player_view().sees( pnt ) ) {
+            if( item.is_seed() ) {
+                add_msg( m_warning, _( "Something has crawled out of the %s plants!" ), item.get_plant_name() );
+            } else {
+                add_msg( m_warning, _( "Something has crawled out of the %s!" ), item.tname() );
+            }
         }
     }
 }