Fix Bug #67468  Segfault in highlight_file()/highlight_string() when pgsql.so module is loaded

str_efree() must be used in zend_highlight() and zend_indent() to free
string data assigned to a zval to account for interned strings.
mention 67468 fix in NEWS
testcase for 67468 (only for debug build, as I could not create a stable segfault without loading other exts like pgsql)
Merge branch 'PHP-5.6'

* PHP-5.6:
  testcase for 67468 (only for debug build, as I could not create a stable segfault without loading other exts like pgsql)
  mention 67468 fix in NEWS
  Fix Bug #67468  Segfault in highlight_file()/highlight_string() when pgsql.so module is loaded