Validate type before inserting into Gob

This fixes the crash caused by:
>> make gob! [pane: [make gob! []]]

because this code would cause "count" to be 0, and results in a
zero-sized series, which is now prohibited.

Author: zsx

src/core/t-gob.c

@@ -197,6 +197,8 @@ const REBCNT Gob_Flag_Words[] = {
 				Detach_Gob(VAL_GOB(val));
 				if ((REBINT)index > i) index--;
 			}
+		} else {
+			Trap_Arg(val);
 		}
 	}
 	arg = sarg;