Verify RegisterProfilerManager doesn't overwrite an existing registra…

…tion

Tested:
Add a second registration to test/profiler_manager_test.cc and
verify the test crashes as expected.

src/benchmark.cc

@@ -663,6 +663,10 @@ void RegisterMemoryManager(MemoryManager* manager) {
 }
 
 void RegisterProfilerManager(ProfilerManager* manager) {
+  // Don't allow overwriting an existing manager.
+  if (manager != nullptr) {
+    BM_CHECK_EQ(internal::profiler_manager, nullptr);
+  }
   internal::profiler_manager = manager;
 }