source-map known bugs

Here's an example test case: ```js // 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(); }; ``` ```js // 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.