Add a missing null pointer check.

util/mac/service_management.cc

Index: util/mac/service_management.cc
diff --git a/util/mac/service_management.cc b/util/mac/service_management.cc
index 512a6059a9b422318e89df71a00de36fd740d2db..0e19114484b6eb147a0985a3afbadb536a03dc66 100644
--- a/util/mac/service_management.cc
+++ b/util/mac/service_management.cc
@@ -129,6 +129,10 @@ pid_t ServiceManagementIsJobRunning(const std::string& label) {
   }
 
   launch_data_t pid = launch_data_dict_lookup(dictionary, LAUNCH_JOBKEY_PID);
+  if (!pid) {
+    return 0;
+  }
+
   if (launch_data_get_type(pid) != LAUNCH_DATA_INTEGER) {
     return 0;
   }