Hook results may redundantly merge (unverified)

The following code looks suspicious/buggy to me, but I haven't tested/verified the problem:

https://github.com/civicrm/civicrm-core/blob/2efcf0c212127e3281834edf26b64d2bbd5e0bb5/CRM/Utils/Hook.php#L169

Suppose we have:
1. $civiModules == array('a','b')
2. $fnName == 'example'
3. function_exists('a_example') == true
4. function_exists('b_example') == false
5. a_example() returns array('first')

It looks like the value of $fResult would"leak" between iterations – ie after calling a_example(), we would set $fResult and merge it in. On the next iteration, $fResult would still be set and would get merged in a second time (even though there's no b_example()).

This probably hasn't been a big problem because most of our hooks don't return values – instead, we usually pass in alterable arrays (array-references) and ignore return values. Never-the-less, I believe some hooks have return values.

Note: In working on this, it would make sense to add more tests to CRM_Utils_HookTest.