Add debug flag to command line interpretation.

extree.c

@@ -18,13 +18,18 @@
    to go away. Hopefully the compiler will realize when it's dead, and
    eliminate it. */
 
-static void print_tree(
+void print_tree(
     FILE *printfile,
     EX_TREE *tp,
     int depth)
 {
     SYMBOL         *sym;
 
+    if (tp == NULL) {
+        fprintf(printfile, "(null)");
+        return;
+    }
+
     switch (tp->type) {
     case EX_LIT:
         fprintf(printfile, "%o", tp->data.lit & 0177777);