[ot-font] Fix hmtx table length checking, *again* Exactly the same problem that I fixed in 63ef0b41dc48d6112d1918c1b1de9de8ea90adb5 I rewrote the table checking yesterday in 67f8821fb25d9bd55719f5e29a582ae1af4b02b3 and introduced the exact same issue again. :( Good thing we have ongoing fuzzing going now. Was discovered immediately by libFuzzer. Thanks kcc! https://github.com/behdad/harfbuzz/issues/139#issuecomment-153449473 Fixes https://github.com/behdad/harfbuzz/issues/156