Add Buffer::copy_from(GWBUF*)

This makes it easier to create a Buffer from GWBUF that the API functions
pass as arguments.

include/maxscale/buffer.hh

@@ -423,9 +423,22 @@ public:
      * @see Buffer::operator =
      */
     bool copy_from(const Buffer& rhs)
+    {
+        return copy_from(rhs.m_pBuffer);
+    }
+
+    /**
+     * Clone a GWBUF and free the current buffer
+     *
+     * @param buf Buffer to clone
+     *
+     * @return True if buffer was copied
+     *
+     * @attention  Invalidates all iterators.
+     */
+    bool copy_from(GWBUF* pBuffer)
     {
         bool copied = true;
-        GWBUF* pBuffer = rhs.m_pBuffer;
 
         if (pBuffer)
         {