Fix crash when no package root argument is specified. (Issue 18935).

runtime/bin/main.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include <stdlib.h>
 #include <string.h>
 #include <stdio.h>
 
 #include "include/dart_api.h"
 #include "include/dart_debugger_api.h"
@@ skipping 373 matching lines @@
   while (i < argc) {
     if (ProcessMainOptions(argv[i])) {
       i++;
     } else {
       // Check if this flag is a potentially valid VM flag.
       const char* kChecked = "-c";
       const char* kPackageRoot = "-p";
       if (strncmp(argv[i], kPackageRoot, strlen(kPackageRoot)) == 0) {
         if (!ProcessPackageRootOption(argv[i] + strlen(kPackageRoot))) {
           i++;
-          if (!ProcessPackageRootOption(argv[i])) {
+          if ((argv[i] == NULL) || !ProcessPackageRootOption(argv[i])) {
             Log::PrintErr("Invalid option specification : '%s'\n", argv[i - 1]);
             i++;
             break;
           }
         }
       } else if (strncmp(argv[i], kChecked, strlen(kChecked)) == 0) {
         vm_options->AddArgument("--checked");
       } else if (!IsValidFlag(argv[i], kPrefix, kPrefixLen)) {
         break;
       }
@@ skipping 731 matching lines @@
   exit(Process::GlobalExitCode());
 }
 
 }  // namespace bin
 }  // namespace dart
 
 int main(int argc, char** argv) {
   dart::bin::main(argc, argv);
   UNREACHABLE();
 }