Backported msgpack#441 to cpp-1.4.

Fixed a pointer operation problem at msgpack::zone::chunk_list::clear().
It was only happened on C++03.

Author: redboltz

erb/cpp03_zone.hpp.erb

@@ -125,6 +125,7 @@ class zone {
                     ::free(c);
                     c = n;
                 } else {
+                    m_head = c;
                     break;
                 }
             }

include/msgpack/detail/cpp03_zone.hpp

@@ -125,6 +125,7 @@ class zone {
                     ::free(c);
                     c = n;
                 } else {
+                    m_head = c;
                     break;
                 }
             }