lib, pimd, zebra: Provide some insurance against reading bad stream data

lib, pimd, zebra: Provide some insurance against reading bad stream data

This patch does two things:

1) Ensure the decoding of stream data between pim <-> zebra is properly
decoded and we don't read beyond the end of the stream.

2) In zebra when we are freeing memory alloced ensure that we
actually have memory to delete before we do so.

Ticket: CM-27055
Signed-off-by: Satheesh Kumar K <sathk@cumulusnetworks.com>
Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>