crash on explicit return from s///e

crash on explicit return from s///e

RT #130188

In

    sub f {
        my $x = 'a';
        $x =~ s/./return;/e;
    }

the 'return' triggers popping any contexts above the subroutine context:
in this case, a CXt_SUBST context. In this case, Perl_dounwind() calls
cx_popblock() for the bottom-most popped context, to restore any saved
vars. However, CXt_SUBST is the one context type which *doesn't* use
'struct block' as part of its context struct union, so you can't
cx_popblock() a CXt_SUBST context.

This commit makes it skip the cx_popblock() in this case.

Bug was introduced by me with v5.23.7-235-gfc6e609.