Mutating `options.format` is unsafe when config is re-used

[robhogan]

Bug report or Feature request?
Bug report

Version (complete output of terser -V or specific git commit)
Confirmed on 5.15.0 and current main (e062dc8)

Description
Terser stores its source map output on the input config object, under options.format.source_map. The returned value includes a getter for a map property, which uses options.format.source_map. If options.format.source_map changes between the minify result and the use of the getter, the output may be incorrect.

In particular, a second call to minify using the same config object will clobber source_map.

Failing test

import {minify} from 'terser';

test('subsequent calls do not clobber previous output', async () => {
  const config = {
    format: {},
    sourceMap: true,
  };

  const fooResult = await minify('"foo";', config);
  const barResult = await minify('module.exports = "bar";', config);

  const fooMap = fooResult.map; // Uses a getter which uses config.format.source_map, which was overwritten by the second call.
  const barMap = barResult.map;

  expect(barMap).toEqual(
    '{"version":3,"names":["module","exports"],"sources":["0"],"mappings":"AAAAA,OAAOC,QAAU"}',
  ); // Succeeds - `barMap` is correct
  expect(fooMap).not.toEqual(barMap); // <-- Fails - `fooMap` is `barMap`
});

Expected result
Test passes - reusing a config object should not affect previous output.