avoid leaks when calling mg_set() in leave_scope()

avoid leaks when calling mg_set() in leave_scope()

In leave_scope() in places like SAVEt_SV, it does stuff like

    if (SvSMAGICAL(...))
        mg_set(...)
    SvREFCNT_dec_NN(ARG0_SV)

If mg_set() dies (e.g. it calls STORE() and STORE() dies), then ARG0_SV
would leak. Fix this by putting ARG0_SV back in the save stack in this
case.

A similar thing applies to SAVEt_AV and SAVEt_HV, but I couldn't
think of a simple test for those, as tied array and hashes don't have
set magic (just RMG).

Also, SAVEt_AV and SAVEt_HV share a lot of common code, so I made
SAVEt_HV goto into the SAVEt_AV code block for the common part.