mg.c: Use SvREFCNT_dec_NN

Using SvREFCNT_dec_NN in a couple of places eliminates needless
null checks.
Use SvREFCNT_dec_NN in one place in mro.c
perldiag: Re-correct spelling of copiable

Sorry, but ‘copyable’ is not a word in English.  It is spelt with an
I, not a Y.  I have already corrected this before, but 50a39ba4 just
‘corrected’ it.
[perl #118159] Make PVs take precedence in SvTRUE

Commit 4bac9ae4 (probably inadvertently) changed SvTRUE to treat an SV
with any of PVX, IVX or NVX having a true value as true.

Traditionally, truth was based solely on stringification. The examina-
tion of the SvIVX and SvNVX slots was for those cases where there was
no string already and it could be deduced from IVX or NVX whether it
would stringify as "0" or no (bugs with -0 aside).

This changes things back to the way they have ‘always’ been.