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.

Beginne in source-map-support.js bei retrieveFile() und retrieveSourceMapURL() und verfolge anschließend den in der Issue beschriebenen Cache-Pfad von mapSourcePosition(). Reproduziere die Suche mit einem großen Bundle, das keine Source Map hat, und überprüfe, dass sein Inhalt nach der fehlgeschlagenen Suche nicht in fileContentsCache zurückbehalten wird.

Vom Indexierungsmodell aus dem Issue-Text verfasst.