use correct dumb variable name

src/interpreter.c

@@ -109,7 +109,7 @@ next:
             void* ptr = (void*) tape_ptr(tape);
             memcpy(&old, ptr, sizeof(Tape*));
             memset(ptr, 0, sizeof(Tape*));
-            tape_free(tape);
+            tape_free(old);
             free(old);
             break;
         }