Fix broken checks in IsValidPartOfMemberNameUtf8Slow.

GetUtf16FromUtf8 returns a surrogate pair only if it encounters
a 4-byte UTF sequence. Three byte UTF sequences will only return the
first or second half of a pair so we need to check for that
explicitly.

bug: 20844537

Change-Id: Icb660fae77ac8a852fc768e6c1cd5766117e68e4
3 files changed