* xmlschemas.c: Fixed a segfault: the instance document was
  still tried to be validated, if the schema, dynamically
  acquired using XSI was invalid, thus mangled. The
  validation will stop (or rather won't validate) now in
  such a case. The schema parser error code will be set
  on the validion context now; this is somehow not nice,
  but it assures that the validation context indicates an
  error in there was a parser error.