net: coap: Improve options parsing helper functions

Options parsing helpers functions unable to handle the malformed
packets and dropping it. Improved parsing functionality to handle
malformed packets. Also payload marker is not mandatory in CoAP
packets when there is no payload. Exit gracefully when CoAP
packet contains only options.

Fixes #4396.

Coverity-ID: 178060

Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>