Excessive memory consumption when handling large bundles without source maps

**Environment:**

- Node.js with source-map-support
- several large JavaScript bundles (5–7 MB each)
- no source maps available for the bundles

**Problem:**

When an error is thrown in a large bundle, the source-map-support library unnecessarily retains the full file contents in memory. The file is loaded solely to check for a source map, but its contents are cached even after this one‑time operation. This results in excessive memory usage, especially problematic with multiple large bundles.

**Steps to reproduce:**

1. An error is thrown in a bundle (e.g., index.js)
2. `Error.prepareStackTrace()` is called
3. The call chain proceeds as follows: `wrapCallSite()` → `mapSourcePosition()` → `retrieveSourceMap("index.js")` → `retrieveSourceMapURL("index.js")` → `retrieveFile("index.js")`
4. The default `retrieveFile` handler loads the entire index.js file from disk and stores it in the `fileContentsCache` ([source](https://github.com/evanw/node-source-map-support/blob/master/source-map-support.js#L129))
5. `retrieveSourceMapURL()` fails to find a source map for the file
6. `mapSourcePosition()` stores an empty source map in its internal cache ([source](https://github.com/evanw/node-source-map-support/blob/master/source-map-support.js#L235))
7. The full contents of index.js remain in the `fileContentsCache` indefinitely, consuming memory

**Expected behavior:**

File contents should be used only once to check for the presence of a source map, and then either not cached at all or discarded immediately after the check. There is no need to retain large bundle files in memory after this operation.

**Actual behavior:**

Large bundle files (5–7 MB) are retained in the `fileContentsCache` after the first error, leading to unnecessary increase in memory footprint.

**Proposed solution:**

Avoid caching when no source map is found: skip storing file contents in `fileContentsCache` if the source map lookup fails.

Start in source-map-support.js at retrieveFile() and retrieveSourceMapURL(), then follow the mapSourcePosition() cache path described in the issue. Reproduce the lookup with a large bundle that has no source map and verify that its contents are not retained in fileContentsCache after the failed lookup.

Written by the indexing model from the issue text.