fix(vm): missing check for thread status after executing hats and typo of stackFrames in retireThread

[alexcui03]

Resolves

Resolves #685

• Typo of thread.stackFrames in Sequencer.retireThread, leading to uncleared stack frames.
• Thread status should be checked before invoking Thread.goToNextBlock in startHats, since the thread maybe done after calling execute and thead.goToNextBlock() will raise an error if the above typo is fixed.

Proposed Changes

• Fix typo of thread.stackFrames in Sequencer.retireThread.
• Check thread status before calling thead.goToNextBlock(). Thread will only go to next block iff it hasn't been done.
• Allow param of reuseStackForNextBlock to be null in jsdoc.

Reason for Changes

Stackframes is not cleared when thread is retired due to typo error. To fix that typo, thread status must be checked before invoking thread.goToNextBlock() in startHats. Previously, when calling goToNextBlock, thread.stack is empty and thread.stackFrames is not empty, then the following code:

scratch-editor/packages/scratch-vm/src/engine/thread.js

Lines 256 to 259 in a54b373

	reuseStackForNextBlock (blockId) {
	this.stack[this.stack.length - 1] = blockId;
	this.stackFrames[this.stackFrames.length - 1].reuse();
	}

will execute as normal, since thread.stack[-1] = null; is valid in javascript. It will assign null value to property '-1', and thread.stack.length will keep 0.

Test Coverage

pass vm/integration/stack-click.js

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[alexcui03]

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