pstatclient: Never pass nullptr to memcpy

Even though the only time this happened was when the size was 0,
it's still undefined to pass memcpy a nullptr.

panda/src/pstatclient/pStatClient.cxx

@@ -1056,7 +1056,9 @@ add_collector(PStatClient::Collector *collector) {
     // the lock.
     int new_collectors_size = (_collectors_size == 0) ? 128 : _collectors_size * 2;
     CollectorPointer *new_collectors = new CollectorPointer[new_collectors_size];
-    memcpy(new_collectors, _collectors, _num_collectors * sizeof(CollectorPointer));
+    if (_collectors != nullptr) {
+      memcpy(new_collectors, _collectors, _num_collectors * sizeof(CollectorPointer));
+    }
     AtomicAdjust::set_ptr(_collectors, new_collectors);
     AtomicAdjust::set(_collectors_size, new_collectors_size);
 
@@ -1091,7 +1093,9 @@ add_thread(PStatClient::InternalThread *thread) {
     // the lock.
     int new_threads_size = (_threads_size == 0) ? 128 : _threads_size * 2;
     ThreadPointer *new_threads = new ThreadPointer[new_threads_size];
-    memcpy(new_threads, _threads, _num_threads * sizeof(ThreadPointer));
+    if (_threads != nullptr) {
+      memcpy(new_threads, _threads, _num_threads * sizeof(ThreadPointer));
+    }
     // We assume that assignment to a pointer and to an int are each atomic.
     AtomicAdjust::set_ptr(_threads, new_threads);
     AtomicAdjust::set(_threads_size, new_threads_size);