Re-examine the SkFontArguments::VariationPosition code regarding life time and zero length input

[HinTak]

See
#297 (comment)

[HinTak]

The setvariationpositioncoordinates method needs a zero length check to avoid setting the coords to garbage and potentially segfault there, and it also needs a keep_alive policy when used in the set property usage. And in the constructor too!