Description:
------------
spl_autoload_call initializes retval to null at the start of the function, but 
does not reinitialize it to null after destroying the return value of each 
autoloader call. As a result, if a subsequent autoloader call does not have any 
return value, then the old dangling pointer is used, resulting in a null pointer 
reference and a segmentation fault, bus error or other entertaining symptom 
depending on the time of day.

Many common autoloaders, such as the Symfony autoloaders, always return true or 
false depending on whether they load a class, even though the documentation for 
spl_autoload_register does not call for this at all. This is probably because 
the developers learned the hard way that autoloaders won't play nice together 
unless they return something due to this bug.

A good example of an autoloader that does trigger this bug is the one provided 
with the Amazon AWS standard library for PHP. Their implementation does not 
return a value, so PHP segfaults (or similar) if it is later in the chain of 
autoloaders.

This bug can be fixed as follows:

if (retval) {
  zval_ptr_dtor(&retval);
}

Becomes:

if (retval) {
  zval_ptr_dtor(&retval);
  retval = NULL;
}

Patch attached.


Expected result:
----------------
Multiple autoloaders play nice.

Actual result:
--------------
If an autoloader other than the first one has no return value a PHP crash takes 
place due to a dangling pointer to a destroyed value.