agg: avoid writting beyond the array size

Potentially trying to add two new items to an array without enough room left.

CID 991455

src/libs/agg/src/agg_vcgen_dash.cpp

@@ -54,7 +54,7 @@ namespace agg
     //------------------------------------------------------------------------
     void vcgen_dash::add_dash(double dash_len, double gap_len)
     {
-        if(m_num_dashes < max_dashes)
+        if(m_num_dashes < max_dashes - 1)
         {
             m_total_dash_len += dash_len + gap_len;
             m_dashes[m_num_dashes++] = dash_len;