ParseXS: refactor: undef cur_xsub/xbody after use

Currently, the fields cur_xsub and cur_xbody of ExtUtils::ParseXS track
the current xsub and body nodes during parsing. This commit undefs them
immediately after use so that they can't be inadvertently used
elsewhere. The fixups in the previous commit were all discovered
by this undeffing.

Author: iabyn

dist/ExtUtils-ParseXS/lib/ExtUtils/ParseXS.pm

@@ -635,6 +635,7 @@ EOM
     $xsub->parse($self)
       or next PARAGRAPH;
     $_ = shift @{$self->{line}};
+    undef $self->{cur_xsub};
 
     $xsub->as_code($self);
     $self->{seen_an_XSUB} = 1; # encountered at least one XSUB

dist/ExtUtils-ParseXS/lib/ExtUtils/ParseXS/Node.pm

@@ -353,6 +353,7 @@ sub parse {
         my $xbody = ExtUtils::ParseXS::Node::xbody->new();
         $pxs->{cur_xbody} = $xbody;
         $xbody->parse($pxs);
+        undef $pxs->{cur_xbody};
 
         if (defined $case) {
             # make the xbody a child of the CASE