Previously, the default RSA_METHOD was NULL until the first RSA structure was

Previously, the default RSA_METHOD was NULL until the first RSA structure was
initialised, at which point an appropriate default was chosen. This meant a
call to RSA_get_default_method might have returned FALSE.

This change fixes that; now any called to RSA_new(), RSA_new_method(NULL), or
RSA_get_default_method() will ensure that a default is chosen if it wasn't
already.