Remove unnecessary null pointer checks.

Some functions perform input parameter validation before their implementations will try further actions with side effects. Some calling functions perform similar safety checks.

Functions which release a system resource are occasionally documented in the way that they tolerate the passing of a null pointer for example. There is not a need because of this fact that a function caller repeats a corresponding check.

[A semantic patch approach](http://coccinelle.lip6.fr/ "Source code transformation engine") can show some changes that are relevant here.

This update suggestion corresponds to [an other bug report](https://sourceforge.net/p/mlt/bugs/216/ "Remove unnecessary null pointer checks.").