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

(cherry picked from commit 3ba8671d)

Change-Id: I2e2a4f9f736cd11050a2b634b3bb27b75a0ee0ba