Ignore up to 4 bytes excess in kern subtables
Ignore up to 4 bytes excess in kern subtables

Fixes https://github.com/behdad/fonttools/issues/314

Previously the warning message was wrong (probably a regression)
as it was reporting the the length of all kern data as "excess".
Fixing that, I see 4 bytes excess in Calibri.  Up to 4 is alright,
since many compilers add padding and wrongly add 4 instead of 0
sometimes.

fonttools/fonttools,googlefonts/fonttools