fix: better handle stdin in sources (#681)

lib/loader.js

@@ -81,12 +81,21 @@ function sassLoader(content) {
       // Since we don't know the final filename in the webpack build chain yet, it makes no sense to have it.
       // eslint-disable-next-line no-param-reassign
       delete result.map.file;
-      // The first source is 'stdin' according to node-sass because we've used the data input.
+      // One of the sources is 'stdin' according to dart-sass/node-sass because we've used the data input.
       // Now let's override that value with the correct relative path.
       // Since we specified options.sourceMap = path.join(process.cwd(), "/sass.map"); in normalizeOptions,
       // we know that this path is relative to process.cwd(). This is how node-sass works.
       // eslint-disable-next-line no-param-reassign
-      result.map.sources[0] = path.relative(process.cwd(), resourcePath);
+      const stdinIndex = result.map.sources.findIndex(
+        (source) => source.indexOf('stdin') !== -1
+      );
+
+      if (stdinIndex !== -1) {
+        result.map.sources[stdinIndex] = path.relative(
+          process.cwd(),
+          resourcePath
+        );
+      }
       // node-sass returns POSIX paths, that's why we need to transform them back to native paths.
       // This fixes an error on windows where the source-map module cannot resolve the source maps.
       // @see https://github.com/webpack-contrib/sass-loader/issues/366#issuecomment-279460722