Check buffer length (#668)

* Check buffer len

* Update execute.ts

* Format

src/execute.ts

@@ -1,6 +1,7 @@
 import {exec} from '@actions/exec'
+import buffer from 'buffer'
 
-let output: string
+let output = ''
 
 /** Wrapper around the GitHub toolkit exec command which returns the output.
  * Also allows you to easily toggle the current working directory.
@@ -29,5 +30,7 @@ export async function execute(
 }
 
 export function stdout(data: any): string | void {
+  if (output.length < buffer.constants.MAX_STRING_LENGTH) {
     output += data.toString().trim()
+  }
 }