fixed bug introduced during OOM fixup causing problems with default

* SAX2.c: fixed bug introduced during OOM fixup causing problems
  with default namespace when a named prefix with the same href
  was present (reported on the mailing list by Karl Eichwalder.
* xmlstring.c: modified xmlCheckUTF8 with suggested code from
  Julius Mittenzwei.
* dict.c: added a typecast to try to avoid problem reported by
  Pascal Rodes.
5 files changed