preprocessor: Initialize a potentially uninitialized variable.

preprocessor: Initialize a potentially uninitialized variable.

My current reading of the relevant static functions suggests that last
is never used without being uninitialized, (we only use it if the
expansion function returned non-NULL and the expansion functions
always set it before returning non-NULL).

Apparently gcc isn't coming to the same conclusion. Initializing this
to NULL nicely quites gcc and will guarantee a nice, early segfault if
my anaylsis turns out to be wrong.