Use g_clear_* helpers where useful

That is:

* when the variable is set to `NULL` right after freeing
* when the variable is checked for non-`NULL` before freeing
* when the variable is a global (because they should be set to `NULL`, even if we don't really claim that things can be re-init'd)

Testing Done:
Compiled, and ran tests in valgrind.

Reviewed at https://reviews.imfreedom.org/r/2369/