Avoid possible free of NULL pointer on Windows

Author: mdboom

cuda_core/cuda/core/system/_system_events.pyx

@@ -81,13 +81,15 @@ cdef class RegisteredSystemEvents:
 
         initialize()
 
+        self._event_set = 0
         self._event_set = nvml.system_event_set_create()
         # If this raises, the event needs to be freed and this is handled by
         # this class's __dealloc__ method.
         nvml.system_register_events(event_bitmask, self._event_set)
 
     def __dealloc__(self):
-        nvml.system_event_set_free(self._event_set)
+        if self._event_set != 0:
+            nvml.system_event_set_free(self._event_set)
 
     def wait(self, timeout_ms: int = 0, buffer_size: int = 1) -> SystemEvents:
         """