unchecked pemalloc() return values

I fixed http://bugs.php.net/bug.php?id=32974 this morning, which was
the result of 3 things going wrong at once.

One of the problems was that the zend_llist functions don't check the
return value from pemalloc(). With this particular bug, malloc()
would fail, even though it was not a low memory condition, and that
would cause a segv when populating the NULL element.

I suspect we have a whole bunch of places where we don't check
pemalloc() return values; the question is, are we concerned enough
about them to add in a NULL check and raise an E_ERROR?

--Wez.