Fix requiredFeature stage handling logic

Fix requiredFeature stage handling logic

Originally the way Jonathan had written this was correct in
"continue"ing:

https://github.com/jfkthame/harfbuzz/commit/35e28c7a733eaffcd9f062b18d7db9fbb3d990fc#diff-ead86a33a5cc9ad7f6e6381031a0baddR199

When I rewrote his patch, I messed it up:

https://github.com/behdad/harfbuzz/commit/da132937989acb4d8ca9bd41c79f98750e7dda30#diff-ead86a33a5cc9ad7f6e6381031a0baddR209

the intended behavior was NOT to set found=TRUE and NOT to continue.
This was resulting in feature_index[table_index] being left unset.
Oops!