Fix mxs::Buffer::copy_from

The code failed to compile when the function is used with a warning that
`pBuffer` was used without initialization. This makes sense as the first
conditional block re-declares the same parameter.

include/maxscale/buffer.hh

@@ -429,7 +429,7 @@ public:
 
         if (pBuffer)
         {
-            GWBUF* pBuffer = gwbuf_clone(pBuffer);
+            pBuffer = gwbuf_clone(pBuffer);
 
             if (!pBuffer)
             {