PHP: fixed crash when calling module functions in OPcache preload.

In PHP, custom fastcgi_finish_request() and overloaded chdir() functions can be
invoked by an OPcache preloading script (it runs when php_module_startup() is
called in the app process setup handler).  In this case, there was no runtime
context set so trying to access it caused a segmentation fault.

This closes #602 issue on GitHub.

docs/changes.xml

@@ -45,6 +45,13 @@ appeared in 1.26.0.
 </para>
 </change>
 
+<change type="bugfix">
+<para>
+a segmentation fault occurred in the PHP module if chdir() or
+fastcgi_finish_request() was called in the OPcache preloading script.
+</para>
+</change>
+
 </changes>