net: zoap: Fix memory overflow issue

If CoAP .well-known/core services list is bigger than single fragment
then current helper functions overwrites beyond fragment space. Which
corrupted whole stack. Right now sending response in multiple fragments
but preferred way is send response in block by block. This should
overcome packet loss across mesh scenarios. Recommended feature will
be supported with later patches.

Change-Id: I30ca55bde2516d80b3583731241ad295799c6614
Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>