No-op iree_event_pool_free.

runtime/src/iree/base/internal/event_pool.c

diff --git a/runtime/src/iree/base/internal/event_pool.c b/runtime/src/iree/base/internal/event_pool.c
index fde006b..3752846 100644
--- a/runtime/src/iree/base/internal/event_pool.c
+++ b/runtime/src/iree/base/internal/event_pool.c
@@ -66,6 +66,7 @@
 }
 
 void iree_event_pool_free(iree_event_pool_t* event_pool) {
+  if (!event_pool) return;
   iree_allocator_t host_allocator = event_pool->host_allocator;
   IREE_TRACE_ZONE_BEGIN(z0);