Fix invalid free of Net::SSLeay::ERROR_WANT_WRITE (95)

This is caused by an autoloaded constant, which does not exist
(Net::SSLeay::ERROR_WANT_WRITE) inside Net::SSLeay::AUTLOAD and is thus
directed to gv_autoload, which required this string to be dynamic.

Added an -O3 exception in save_pv_or_rv, similar to warnings::Bits.