Tools/MMapGenerator: Fix an edge case of infinite loop

Fix a (pseudo) infinite loop happening when generating mmaps for single tile maps (like 249) but with vmaps not present in the correct folder. This would trigger a 18446744065119600000 (uint32 max ^ 2) iterations loop.

src/tools/mmaps_generator/MapBuilder.cpp

@@ -215,10 +215,12 @@ namespace MMAP
     /**************************************************************************/
     void MapBuilder::getGridBounds(uint32 mapID, uint32 &minX, uint32 &minY, uint32 &maxX, uint32 &maxY) const
     {
-        maxX = std::numeric_limits<uint32>::max();
-        maxY = std::numeric_limits<uint32>::max();
-        minX = 0;
-        minY = 0;
+        // min and max are initialized to invalid values so the caller iterating the [min, max] range
+        // will never enter the loop unless valid min/max values are found
+        maxX = 0;
+        maxY = 0;
+        minX = std::numeric_limits<uint32>::max();
+        minY = std::numeric_limits<uint32>::max();
 
         float bmin[3] = { 0, 0, 0 };
         float bmax[3] = { 0, 0, 0 };