further fix to SvUPGRADE

further fix to SvUPGRADE

The change to SvUPGRADE introduced by 463ea2290a54e a few commits ago
to silence a warning with clang, broke g++ builds instead. Here's
a second attempt to keep everyone happy.

Basically it avoids warnings from all of gcc, g++ and clang for the two
constructs

    SvUPGRADE(...);
    (void)SvUPGRADE(...);

But still breaks

    if (!SvUPGRADE(...) { croak(...); }

which I don't care about.