Fix cv-to-gv assignment to use CvPROTO

Fix cv-to-gv assignment to use CvPROTO

The SvPVX field of a XS AUTOLOAD sub can contain both the prototype
and the name of an AUTOLOADed sub.  The CvPROTO macro knows where in
the buffer to find the prototype.  All code that reads the prototype
should use it.  When I introduced it with commit 8fa6a4095, one code
path was missed: *regular_prototyped_sub = \&prototyped_XS_AUTOLOAD
was using the sub name of the rhs, instead of the prototype, in the
prototype check.