Remove `this` null pointer comparisons

It appears the standard specifies that `this` being null is undefined
behavior. Compilers may choose to optimize taking UB into account; in
particular, the null pointer check in question can be optimized out.
clang 3.5 has a new warning flag -Wtautological-pointer-compare which
warns against that particular case, among others.

http://llvm.org/releases/3.5.0/tools/clang/docs/ReleaseNotes.html

This fixes the following warnings with clang 3.5:

  src/actions/../game_data.hpp:66:5: error: 'this' pointer cannot be null in well-defined C++ code; comparison may be assumed to always evaluate to true [-Werror,-Wtautological-undefined-compare]
  src/actions/../game_data.hpp:74:5: error: 'this' pointer cannot be null in well-defined C++ code; comparison may be assumed to always evaluate to true [-Werror,-Wtautological-undefined-compare]
  src/actions/../game_data.hpp:130:5: error: 'this' pointer cannot be null in well-defined C++ code; comparison may be assumed to always evaluate to true [-Werror,-Wtautological-undefined-compare]