Original position lookup in an indexed source map can fail due to failure to find the right section

[takikawa]

Here's an example test case:

// add to test/test-source-map-consumer

exports["test a mapping at the boundary of indexed source map offset"] =
  async function (assert) {
    const map = await new SourceMapConsumer(util.indexedTestMapAtOffsetBoundary);
    util.assertMapping(1, 0, "/the/root/one.js", 1, 0, null, null, map, assert);
    map.destroy();
  };

// add to test/util.js 

// This mapping is for testing a case where the mapped position is at the
// section offset.
exports.indexedTestMapAtOffsetBoundary = {
  version: 3,
  file: "min.js",
  sections: [
    {
      offset: {
        line: 0,
        column: 0,
      },
      map: {
        version: 3,
        sources: ["one.js"],
        sourcesContent: [
          "ONE.foo = function (bar) {\n   return baz(bar);\n };",
        ],
        names: ["bar", "baz"],
        mappings: "AAAA",
        file: "min.js",
        sourceRoot: "/the/root",
      },
    },
  ],
};

Currently it would fail like this:

FAILED ./test-source-map-consumer: test a mapping at the boundary of indexed source map offset!
AssertionError [ERR_ASSERTION]: Incorrect line, expected 1, got null
    at Object.assertMapping (/home/asumu/source-map/test/util.js:322:12)
    at exports.test a mapping at the boundary of indexed source map offset (/home/asumu/source-map/test/test-source-map-consumer.js:2190:10)
    at async run (/home/asumu/source-map/test/run-tests.js:21:11)

In this test, the index map has a section at offset line 0, column 0. The section has a single mapping from 0, 0 -> 0, 0. This should behave the same as a regular source map with just that one section.

However, the test actually fails because the section lookup fails. I think this is actually due to an off-by-one error in the section lookup code, which I'll submit a PR for.