Fix uninitialized (pointer) member variables

As the headline suggests, this pull request fixes some uninitialized member variables (mostly pointers) in various classes by initializing them to NULL.

For reasons why this should be done, see http://cwe.mitre.org/data/definitions/457.html.
>In some languages such as C and C++, stack variables are not initialized by default. They generally contain junk data with the contents of stack memory before the function was invoked. An attacker can sometimes control or read these contents. In other languages or conditions, a variable that is not explicitly initialized can be given a default value that has security implications, depending on the logic of the program. The presence of an uninitialized variable can sometimes indicate a typographic error in the code.