Fixed a difference in processing of xsl:value-of and xsl:text wrt to

* libxslt/transform.c: Fixed a difference in processing of
  xsl:value-of and  xsl:text wrt to merging of strings of
  output-escaped text-nodes. This difference made the
  optimized string-merging mechanism run out of sync, which
  led to segfaults in subsequent string reallocations. See
  bug #343411, reported by Grzegorz Kaczor.
* xslt.c: Added check for ctxt->internalized when we parse
  xsl:text and internalize the strings.