two defensive changes from Tobias Stoeckmann via GHPR287

two defensive changes from Tobias Stoeckmann via GHPR287

enforce stricter invarient for sshbuf_set_parent() - never allow
a buffer to have a previously-set parent changed.

In sshbuf_reset(), if the reallocation fails, then zero the entire
buffer and not the (potentially smaller) default initial alloc size.