Fixed GPF in fast-import caused by unterminated linked list.

fast-import was encounting a GPF when it ran out of free tree_entry
objects but didn't know this was the cause because the last
tree_entry wasn't terminated with a NULL pointer.  The missing NULL
pointer occurred when we allocated additional entries via xmalloc
but didn't set the last tree_entry's "next" pointer to NULL.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

fast-import.c

@@ -520,10 +520,11 @@ static struct tree_entry* new_tree_entry()
 		unsigned int n = tree_entry_alloc;
 		total_allocd += n * sizeof(struct tree_entry);
 		avail_tree_entry = e = xmalloc(n * sizeof(struct tree_entry));
-		while (n--) {
+		while (n-- > 1) {
 			*((void**)e) = e + 1;
 			e++;
 		}
+		*((void*)e) = NULL;
 	}
 
 	e = avail_tree_entry;