Keep a local copy of the string in the envelope, so that the data does not go out of scope

include/envelope.h

@@ -18,6 +18,12 @@ namespace AMQP {
 class Envelope : public MetaData
 {
 protected:
+    /**
+     *  The body (only used when string object was passed to constructor
+     *  @var    std::string
+     */
+    std::string _str;
+
     /**
      *  Pointer to the body data (the memory buffer is not managed by the AMQP
      *  library!)
@@ -47,7 +53,7 @@ public:
      *  Constructor based on a string
      *  @param  body
      */
-    Envelope(const std::string &body) : MetaData(), _body(body.data()), _bodySize(body.size()) {}
+    Envelope(const std::string &body) : MetaData(), _str(body), _body(_str.data()), _bodySize(_str.size()) {}
 
     /**
      *  Destructor