Fix "invalid string length" (microsoft#158216)

Fixes microsoft#158204

Author: roblourens

src/vs/workbench/services/search/node/ripgrepTextSearchEngine.ts

@@ -92,7 +92,10 @@ export class RipgrepTextSearchEngine {
 			rgProc.stderr!.on('data', data => {
 				const message = data.toString();
 				this.outputChannel.appendLine(message);
-				stderr += message;
+
+				if (stderr.length + message.length < 1e6) {
+					stderr += message;
+				}
 			});
 
 			rgProc.on('close', () => {