Fix buffers_prefix iterator decrement

include/boost/beast/core/buffers_prefix.hpp

@@ -39,6 +39,7 @@ class buffers_prefix_view
 
     BufferSequence bs_;
     std::size_t size_;
+    std::size_t remain_;
     iter_type end_;
 
     template<class Deduced>
@@ -46,6 +47,7 @@ class buffers_prefix_view
             Deduced&& other, std::size_t dist)
         : bs_(std::forward<Deduced>(other).bs_)
         , size_(other.size_)
+        , remain_(other.remain_)
         , end_(std::next(bs_.begin(), dist))
     {
     }

include/boost/beast/core/impl/buffers_prefix.ipp

@@ -125,7 +125,7 @@ private:
     const_iterator(buffers_prefix_view const& b,
             std::true_type)
         : b_(&b)
-        , remain_(0)
+        , remain_(b.remain_)
         , it_(b_->end_)
     {
     }
@@ -145,6 +145,7 @@ buffers_prefix_view<BufferSequence>::
 setup(std::size_t size)
 {
     size_ = 0;
+    remain_ = 0;
     end_ = boost::asio::buffer_sequence_begin(bs_);
     auto const last = bs_.end();
     while(end_ != last)
@@ -154,6 +155,7 @@ setup(std::size_t size)
         if(len >= size)
         {
             size_ += size;
+            remain_ = size - len;
             break;
         }
         size -= len;
@@ -192,6 +194,7 @@ operator=(buffers_prefix_view&& other) ->
         other.end_);
     bs_ = std::move(other.bs_);
     size_ = other.size_;
+    remain_ = other.remain_;
     end_ = std::next(
         boost::asio::buffer_sequence_begin(bs_),
             dist);
@@ -209,6 +212,7 @@ operator=(buffers_prefix_view const& other) ->
         other.end_);
     bs_ = other.bs_;
     size_ = other.size_;
+    remain_ = other.remain_;
     end_ = std::next(
         boost::asio::buffer_sequence_begin(bs_),
             dist);