dix: fix security and memory issues if calloc is failed

[GermanAizek]

@metux, I think you'll like using pointer checking style, and there's a lot test ptr coverage calloc() function's returned pointers. Using dynamic sanitizers, I found possible calloc() fails leaks.

Duplicate description PR: https://github.com/X11Libre/xserver/pull/79

[GermanAizek]

Continue same changes in exa driver: https://github.com/X11Libre/xserver/pull/85

[metux]

needs rebase.

[metux]

@GermanAizek needs rebase.