Use default member initialization

Author: joto

include/osmium/area/detail/proto_ring.hpp

@@ -86,16 +86,16 @@ namespace osmium {
                 int64_t m_num;
 #endif
 
-                int64_t m_sum;
+                int64_t m_sum = 0;
 
             public:
 
                 explicit ProtoRing(NodeRefSegment* segment) noexcept :
-                    m_min_segment(segment),
+                    m_min_segment(segment)
 #ifdef OSMIUM_DEBUG_RING_NO
-                    m_num(next_num()),
+                    , m_num(next_num())
 #endif
-                    m_sum(0) {
+                    {
                     add_segment_back(segment);
                 }