libwebsockets

coverity: 62123: explicitly check things to guide coverity

Coverity sees we sometimes check if header length is 0 and extrapolates
from that not checking header length return is a violation.  But often
we are OK if the header length is 0 and there is no error return to
check from that.

It also doesn't understand that if we saw a nonzero length for a header,
then we are going to get a non-null simple_ptr() return for sure.  Just
give up and explicitly, unneccessarily check everything so coverity can
stop telling us about it.