src: guard against starting fs watcher twice

This commit adds a CHECK that verifies that the file event watcher is
not started twice, which would be indicative of a bug in lib/fs.js.

PR-URL: #7374
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

src/fs_event_wrap.cc

@@ -88,6 +88,7 @@ void FSEventWrap::Start(const FunctionCallbackInfo<Value>& args) {
 
   FSEventWrap* wrap;
   ASSIGN_OR_RETURN_UNWRAP(&wrap, args.Holder());
+  CHECK_EQ(wrap->initialized_, false);
 
   static const char kErrMsg[] = "filename must be a string or Buffer";
   if (args.Length() < 1)