rt: Don't print a stack with no caller

Author: hexaredecimal

src/main/java/org/piccode/rt/PiccodeException.java

@@ -90,6 +90,7 @@ public void report(boolean die, String kind) {
 			for (int i = 0; i < list.size(); i++) {
 				var _frame = list.get(i);
 				var callSite = _frame.caller;
+				if (callSite == null) continue;
 				var _str = String.format(
 								"[%s:%d:%d]: %s", callSite.file,
 								callSite.line, callSite.column + 1,