[HarfBuzz] fallback kern optimization

[Jonathan Kew]

It's a relatively small thing, but we can get a slight win for the case 
where kerning has been disabled if we add an early return to 
_hb_ot_shape_fallback_kern().

Looks to me like we can also reduce the use of buffer->... there when 
kerning is being applied, which might help a bit (depending how well the 
compiler optimizes stuff.)

These changes seem to yield a small but just-about-measurable 
improvement with old fonts that lack GPOS kerning.

JK
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