Avoid rebuilding the property table when possible in SplFixedArray's gc handler

[nielsdos]

If there is not yet a dynamic property, and there are no class properties, then we know that we don't have to build a properties table.

For this (micro-bench) script:

function x() {
    $fa = new SplFixedArray(1);
    $fa[0] = $fa;
}
for ($i=0;$i<1000000;$i++)
    x();

On an i7-4790:

Benchmark 1: ./sapi/cli/php spl.php
  Time (mean ± σ):     140.9 ms ±   1.2 ms    [User: 137.5 ms, System: 2.7 ms]
  Range (min … max):   138.9 ms … 144.9 ms    21 runs

Benchmark 2: ./sapi/cli/php_old spl.php
  Time (mean ± σ):     162.0 ms ±   3.8 ms    [User: 157.7 ms, System: 3.2 ms]
  Range (min … max):   158.5 ms … 175.0 ms    17 runs

Summary
  ./sapi/cli/php spl.php  ran
    1.15 ± 0.03 times faster than ./sapi/cli/php_old spl.php