Remove bogus ZEND_ASSERT(hash != 0)

Remove bogus ZEND_ASSERT(hash != 0)
There's always a possibility that the has will end up as equal to zero due to overflowing.
See the tests for strings whose hash overflow.
Yes, there's a check for whether the hash already was calculated... and strings with a hash equal to zero always need the hash recalculated, in one of 2^32 respectively 2^64 cases - which should not be a major issue.