Removing uninitialized_var()

I've always thought uninitialize_var() macro was a bit ugly to look at.

The problem with setting pointers to NULL is that it silences the GCC warning, but it now triggers a "dereferencing NULL" warning in Smatch. Uninitialize_var() solved this problem nicely. I wish there were some macro that Smatch could understand.

struct foo *p = SILENCE_GCC;