Fix assertion String.cc:221: "str"

Fix assertion String.cc:221: "str"

This bug can be caused by certificates does not contain a CN field. In this
case the Ssl::ErrorDetail::cn method may return NULL causing this assertion
somewhere inside Ssl::ErrorDetail::buildDetail method, which expects always
a non NULL value from Ssl::ErrorDetail::cn and similar methods.

This patch try to hardening the Ssl::ErrorDetail error formating functions to
avoid always check for NULL values and also avoid sending wrong information
for various certificate fields in the case of an error while extracting the
information from certificate..

This is a Measurement Factory project