MXS-870: Handle non-contiguous session command responses

Session command responses with multiple packets could be spread across
multiple, non-contiguous buffers. If a buffer contained a complete packet
and some extra data but it wasn't contiguous, the debug assertion in
gwbuf_clone_portion would fail. With release builds, it would cause
eventual out-of-bounds memory access when the response would be sent to
the client.

server/core/buffer.c

@@ -384,9 +384,9 @@ GWBUF* gwbuf_clone_all(GWBUF* buf)
 }
 
 
-GWBUF *gwbuf_clone_portion(GWBUF *buf,
-                           size_t start_offset,
-                           size_t length)
+static GWBUF *gwbuf_clone_portion(GWBUF *buf,
+                                  size_t start_offset,
+                                  size_t length)
 {
     GWBUF* clonebuf;