Optimise magic handling in save* and leave_scope

Optimise magic handling in save* and leave_scope

There are several places that have code similar to

    if (SvMAGICAL(sv)) {
        PL_localizing = 2;
        SvSETMAGIC(sv)
        PL_localizing = 0;
    }

The condition is sub-optimal (it should only test for set magic), and the
SvSETMAGIC repeats a similar test.
Other places didn't have the outer condition, so they set PL_localizing
twice even when not needed.