Fix segmentation fault on -k parse (#218)

Fix segmentation fault on -k parse (#218)

When an HTTPS or SSL-Bump port is configured without a cert=
parameter it results in a segmentation fault. Detect that
occurance and add the required FATAL error message instead for
these configurations where cert= is a parameter rather than an
option.

Our project terminology for config settings is;

 "parameter"
 -  a required setting. Print a FATAL error message if missing.

 "option"
 -  an optional setting. Ignored or default value if missing.