fix dereferencing out-of-range iterator (caught by -fsanitize=undefined,integer) (#3867)

include/mapnik/agg_renderer.hpp

@@ -81,18 +81,21 @@ public:
         }
         else
         {
-            position_--;
+            --position_;
             mapnik::fill(*position_, 0); // fill with transparent colour
         }
         return *position_;
     }
+    bool in_range() const
+    {
+        return (position_ != buffers_.end());
+    }
 
     void pop()
     {
-        if (position_ != buffers_.end())
-        {
-            position_++;
-        }
+        // ^ ensure irator is not out-of-range
+        // prior calling this method
+        ++position_;
     }
 
     T & top() const

src/agg/agg_renderer.cpp

@@ -340,7 +340,8 @@ void agg_renderer<T0,T1>::end_style_processing(feature_type_style const& st)
                       -common_.t_.offset(),
                       -common_.t_.offset());
         }
-        if (&current_buffer == &internal_buffers_.top())
+        if (internal_buffers_.in_range()
+            && &current_buffer == &internal_buffers_.top())
         {
             internal_buffers_.pop();
         }