Perl_call_sv move EXTEND into branch that needs it

If we aren't manipulating the stack, dont fetch it, check and possibly
extend it. There is a slight chance this EXTEND was covering up missing
EXTENDs somewhere else in Perl core or CPAN C code, if future bisects or
valgrind reports show that this EXTEND by 1 must always be done, this
patch can be reverted. pp_method_named contains a EXTEND/XPUSH* call,
pp_entersub requires 1 arg on stack so, both sides of the
"if (!(flags & G_METHOD_NAMED))" test will in theory make sure there is
1 free slot on the stack on entry to a SUB or XSUB.

See also
http://www.nntp.perl.org/group/perl.perl5.porters/2015/09/msg231329.html