[perl #118159] Make PVs take precedence in SvTRUE

[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.