Allow non-head use of gwbuf_copy_data

Non-head links in a buffer chain can now be used with gwbuf_copy_data
without triggering the debug assertion.

server/core/buffer.cc

@@ -723,7 +723,6 @@ GWBUF* gwbuf_make_contiguous(GWBUF* orig)
 
 size_t gwbuf_copy_data(const GWBUF* buffer, size_t offset, size_t bytes, uint8_t* dest)
 {
-    validate_buffer(buffer);
     uint32_t buflen;
 
     /** Skip unrelated buffers */