src: fix debugging for multiple categories

Fix parsing of e.g. `NODE_DEBUG_NATIVE=worker,messaging`.

PR-URL: https://github.com/nodejs/node/pull/21422
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

src/env.cc

@@ -537,7 +537,7 @@ void Environment::set_debug_categories(const std::string& cats, bool enabled) {
     if (comma_pos == std::string::npos)
       break;
     // Use everything after the `,` as the list for the next iteration.
-    debug_categories = debug_categories.substr(comma_pos);
+    debug_categories = debug_categories.substr(comma_pos + 1);
   }
 }