* xmlschemas.c xmlstring.c: Fixed a segfault during
  text concatenation when validating a node tree:
  xmlStrncat was called with a @len of -1; but unlike
  xmlStrncatNew, it does not calculate the length
  automatically in such a case (reported by Judy Hay
  on the mailing list).
  Updated the descriptions of the involved string
  functions to note this.