Avoid overflow in get_overmaps_near

In some cases a call to get_overmaps_near was passing a tripoint with z
value INT_MIN.  This led to an overflow when computing the distance
between points.

Instead, just pass the x,y part of the tripoint.

src/overmapbuffer.cpp

@@ -1130,7 +1130,7 @@ std::vector<shared_ptr_fast<npc>> overmapbuffer::get_npcs_near( const tripoint &
                                int radius )
 {
     std::vector<shared_ptr_fast<npc>> result;
-    for( auto &it : get_overmaps_near( p, radius ) ) {
+    for( auto &it : get_overmaps_near( p.xy(), radius ) ) {
         auto temp = it->get_npcs( [&]( const npc & guy ) {
             // Global position of NPC, in submap coordinates
             const tripoint pos = guy.global_sm_location();