[unsafe] Unchecked UCD access

[behdad]

Just for the record. Made using harfbuzz/packtab#6
About 1% speedup.

Comparing before to after
Benchmark                                                                               Time             CPU      Time Old      Time New       CPU Old       CPU New
--------------------------------------------------------------------------------------------------------------------------------------------------------------------
BM_Shape/NotoNastaliqUrdu-Regular.ttf/fa-thelittleprince.txt/harfrust                -0.0138         -0.0138           110           109           110           108
BM_Shape/NotoNastaliqUrdu-Regular.ttf/fa-words.txt/harfrust                          -0.0020         -0.0024           128           128           128           127
BM_Shape/Amiri-Regular.ttf/fa-thelittleprince.txt/harfrust                           -0.0114         -0.0107            46            46            46            45
BM_Shape/NotoSansDevanagari-Regular.ttf/hi-words.txt/harfrust                        -0.0059         -0.0055            28            28            28            28
BM_Shape/Roboto-Regular.ttf/en-thelittleprince.txt/harfrust                          -0.0120         -0.0114             9             9             9             9
BM_Shape/Roboto-Regular.ttf/en-words.txt/harfrust                                    -0.0327         -0.0321            14            13            14            13
BM_Shape/SourceSerifVariable-Roman.ttf/react-dom.txt/harfrust                        -0.0189         -0.0183           103           101           103           101
OVERALL_GEOMEAN                                                                      -0.0138         -0.0135             0             0             0             0