Zend: fix overflow handling bug in non-x86 fast_add_function()

The 'result' argument of fast_add_function() may alias with either
of its operands (or both). Take care not to write to 'result' before
reading op1 and op2.
Merge branch 'PHP-5.4' into PHP-5.5
Merge branch 'PHP-5.5' into PHP-5.6
Merge branch 'PHP-5.6'