Null pointer dereference regression in parse.y

How it was found

Hi,

Just pulled the latest mruby code, and found that some of my fuzzing test cases now crash. Bisected it to commit 227daa881137d5251e03eea0883b9b574a1f064e. Reverting this change no longer causes a crash.

The minimised file causing the crash is:

f ()

Also affects mruby-engine.

Cheers,

Hugh