ASSERT((characters + 1) == (characters + length));
It looks to me like this assertion should be:
ASSERT((characters + 1) == (this->characters() + length));
Created attachment 61085 [details]
From the ChangeLog:
Fixed assertion when sourceExhausted is returned from convertUTF16ToUTF8.
Comment on attachment 61085 [details]
Committed r63016: <http://trac.webkit.org/changeset/63016>