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

runtime/bin/main.cc

Index: runtime/bin/main.cc
===================================================================
--- runtime/bin/main.cc	(revision 36432)
+++ runtime/bin/main.cc	(working copy)
@@ -391,7 +391,7 @@
       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;