fix agg overflows

fix-agg-unsigned-overflow.diff

diff --git a/deps/agg/include/agg_array.h b/deps/agg/include/agg_array.h
index 8d56683..9c7a447 100644
--- a/deps/agg/include/agg_array.h
+++ b/deps/agg/include/agg_array.h
@@ -513,7 +513,7 @@ namespace agg
     //------------------------------------------------------------------------
     template<class T, unsigned S> pod_bvector<T, S>::~pod_bvector()
     {
-        if(m_num_blocks)
+        if(m_num_blocks > 0)
         {
             T** blk = m_blocks + m_num_blocks - 1;
             while(m_num_blocks--)
diff --git a/deps/agg/include/agg_rasterizer_cells_aa.h b/deps/agg/include/agg_rasterizer_cells_aa.h
index 4e1c62a..f35b884 100755
--- a/deps/agg/include/agg_rasterizer_cells_aa.h
+++ b/deps/agg/include/agg_rasterizer_cells_aa.h
@@ -663,14 +663,17 @@ namespace agg
         cell_type*  cell_ptr;
         unsigned nb = m_num_cells >> cell_block_shift;
         unsigned i;
-        while(nb--)
+        if (nb > 0)
         {
-            cell_ptr = *block_ptr++;
-            i = cell_block_size;
-            while(i--) 
+            while(nb--)
             {
-                m_sorted_y[cell_ptr->y - m_min_y].start++;
-                ++cell_ptr;
+                cell_ptr = *block_ptr++;
+                i = cell_block_size;
+                while(i--)
+                {
+                    m_sorted_y[cell_ptr->y - m_min_y].start++;
+                    ++cell_ptr;
+                }
             }
         }
 
diff --git a/deps/agg/include/agg_rasterizer_scanline_aa.h b/deps/agg/include/agg_rasterizer_scanline_aa.h
index 77bc41b..544f76c 100644
--- a/deps/agg/include/agg_rasterizer_scanline_aa.h
+++ b/deps/agg/include/agg_rasterizer_scanline_aa.h
@@ -259,7 +259,8 @@ namespace agg
 
                     if(area)
                     {
-                        alpha = calculate_alpha((cover << (poly_subpixel_shift + 1)) - area);
+                        int cover_shift = cover << (poly_subpixel_shift + 1);
+                        alpha = calculate_alpha(cover_shift - area);
                         if(alpha)
                         {
                             sl.add_cell(x, alpha);
@@ -269,7 +270,8 @@ namespace agg
 
                     if(num_cells && cur_cell->x > x)
                     {
-                        alpha = calculate_alpha(cover << (poly_subpixel_shift + 1));
+                        int cover_shift = cover << (poly_subpixel_shift + 1);
+                        alpha = calculate_alpha(cover_shift);
                         if(alpha)
                         {
                             sl.add_span(x, cur_cell->x - x, alpha);
diff --git a/deps/agg/include/agg_rendering_buffer.h b/deps/agg/include/agg_rendering_buffer.h
index e43899e..14ed414 100644
--- a/deps/agg/include/agg_rendering_buffer.h
+++ b/deps/agg/include/agg_rendering_buffer.h
@@ -186,7 +186,7 @@ namespace agg
 
             T** rows = &m_rows[0];
 
-            while(height--)
+            while(height > 0 && height--)
             {
                 *rows++ = row_ptr;
                 row_ptr += stride;
diff --git a/include/mapnik/vertex_vector.hpp b/include/mapnik/vertex_vector.hpp
index 44fbd78..12c0f92 100644
--- a/include/mapnik/vertex_vector.hpp
+++ b/include/mapnik/vertex_vector.hpp
@@ -77,7 +77,7 @@ public:
         if ( num_blocks_ )
         {
             coord_type** vertices=vertices_ + num_blocks_ - 1;
-            while ( num_blocks_-- )
+            while ( num_blocks_> 0 && num_blocks_-- )
             {
                 ::operator delete(*vertices);
                 --vertices;