Changes

msgb: msgb_copy_resize_c: Fix validation check to avoid memcpy buffer overflow

If msg->data pointer is not allocated at the start of the msgb, (eg.
because it was pull()ed or had some headroom), the existing check
wouldn't catch it and memcpy() would write passed the allocated chunk
(msg->data - msg->_data) bytes.

Change-Id: If4c84162a4e5b44b82813fb58029fae04bd38230