cluster: only assign debug port if in valid range.

See: nodejs#8159

Per above discussion, cluster.fork() currently sends --debug-port to
forked processes regardless of whether debug is enabled, and more
importantly, without any bounds checking. This will rather unexpectedly
crash and Node process that forks enough times.

This patch simply bounds checks --debug-port and doesn't set arg if out
of bounds (V8 requires 1024 < debug-port < 65535). This will prevent
surprises to callers of cluster.fork() while waiting for the debug part
of node to be rewritten as mentioned in issue discussion above.

lib/cluster.js

@@ -307,17 +307,19 @@ function masterInit() {
     workerEnv = util._extend(workerEnv, env);
     workerEnv.NODE_UNIQUE_ID = '' + id;
 
-    for (var i = 0; i < execArgv.length; i++) {
-      var match = execArgv[i].match(/^(--debug|--debug-brk)(=\d+)?$/);
+    if (1024 < debugPort < 65535) {
+      for (var i = 0; i < execArgv.length; i++) {
+        var match = execArgv[i].match(/^(--debug|--debug-brk)(=\d+)?$/);
 
-      if (match) {
-        execArgv[i] = match[1] + '=' + debugPort;
-        hasDebugArg = true;
+        if (match) {
+          execArgv[i] = match[1] + '=' + debugPort;
+          hasDebugArg = true;
+        }
       }
-    }
 
     if (!hasDebugArg)
       execArgv = ['--debug-port=' + debugPort].concat(execArgv);
+    }
 
     return fork(cluster.settings.exec, cluster.settings.args, {
       env: workerEnv,