Initialize CLEANUP_* stack variables with NULL in various places.

Code like:

  CLEANUP_FREE char *buf;
  /* some code which might return early */
  buf = malloc (10);

is a potential bug because the free (*buf) might be called when buf is
an uninitialized pointer.  Initialize buf = NULL to avoid this.

Several of these are bugs, most are not bugs (because there is no
early return statement before the variable gets initialized).

However the compiler can elide the initialization, and even if it does
not the performance "penalty" is miniscule, and correctness is better.