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.

glcpp/glcpp-parse.y

@@ -1312,7 +1312,7 @@ _glcpp_parser_expand_token_list (glcpp_parser_t *parser,
 				 token_list_t *list)
 {
 	token_node_t *node_prev;
-	token_node_t *node, *last;
+	token_node_t *node, *last = NULL;
 	token_list_t *expansion;
 
 	if (list == NULL)