Fix heap leak on soft reset.

[microbit-matt-hillsdon]

Previously flashing in a loop would run out of memory.

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[microbit-matt-hillsdon]

I reproduced with:

      const doFlash = () => {
        simulator.postMessage(
                {
                  kind: "flash",
                  filesystem: {
                    "main.py": new TextEncoder().encode(program.value),
                  },
                },
                "*"
              );
        setTimeout(doFlash, 50);
      }
      doFlash();

...at the end of the JS in demo.html.

[microbit-robert]

LGTM.