InsertUnicodeExtension when called from ApplyUnicodeExtensionToTag cannot fail

[anba]

ApplyUnicodeExtensionToTag, step 10.a

10. If newExtension is not the empty String, then
    a. Let locale be ? InsertUnicodeExtension(locale, newExtension).

But InsertUnicodeExtension can only fail when locale is either a grandfathered or private-use only tag, see InsertUnicodeExtension, step 3.

3. If locale matches the privateuse or the grandfathered production, throw a RangeError exception.

And when called from ApplyUnicodeExtensionToTag, locale is never a grandfathered or private-use only tag, because of ApplyUnicodeExtensionToTag, step 2.

2. If tag matches the privateuse or the grandfathered production, then
   ...
   d. Return result.

[littledan]

Fixed by #66