[minor fix] Replaced a malloc followed by the zeroing of the structure with a calloc.

This offers a little speed boost and, in case someone changes the z_stream structure, there won't be any corruption of the data if variables aren't initialized properly.
[minor fix] Replaced exit(1) with exit(EXIT_FAILURE) for portability.
[minor fix] Replaced exit(1) with exit(EXIT_FAILURE) for portability.
[minor fix] Replaced exit(0) with exit(EXIT_SUCCESS).

They're basically the same thing but it should be done this way for portability.
Merge pull request #146 from OHamm/master

[minor fix] Replaced a malloc followed by the zeroing of the structure with a calloc.