net: zoap: Refactor zoap utilities

Most of the zoap utilities deal with first fragment of buffer chain.
So accessing of buf->frags directly make less usage of stack than
delcaring another variable for frag and its data. Due to code
refactoring couple of lines removed and in between those lines
coverity complained two issues, which is not valid now.

Coverity-CID: 157597
Coverity-CID: 157598

Change-Id: I3035c0625a131d49a4f0250ff0052875d3382544
Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>