S_doparseform() should return void, not OP*, as it should use Perl_die not DIE

S_doparseform() should return void, not OP*, as it should use Perl_die not DIE

a1b950687051c32e added an error condition in S_doparseform() but used DIE(...)
to report it. DIE is defined as C<return Perl_die>, which acts as a hint to the
compiler about the control flow [as Perl_die() never returns], but also forces
the return type to be OP *. Whilst this is appropriate for pp functions, it's
not for S_doparseform() - a1b950687051c32e had to change the return type to OP*
and return NULL, just to appease DIE(). Hence use Perl_die() instead, remove
return statements, and remove the didn't-return-NULL (dead) code from
pp_formline.