Add a null check for suppressed exception.

ScopedExceptionStorage may be used when there is no pending exception.

Test: m
Bug: 233191719
Change-Id: I03c4da8e55a9259e646d2acdacdcf75f107d3f43
diff --git a/runtime/thread.cc b/runtime/thread.cc
index e629b4e..e7c6bec 100644
--- a/runtime/thread.cc
+++ b/runtime/thread.cc
@@ -4613,7 +4613,9 @@
   CHECK(self_->IsExceptionPending()) << *self_;
   ObjPtr<mirror::Throwable> old_suppressed(excp_.Get());
   excp_.Assign(self_->GetException());
-  LOG(WARNING) << message << "Suppressing old exception: " << old_suppressed->Dump();
+  if (old_suppressed != nullptr) {
+    LOG(WARNING) << message << "Suppressing old exception: " << old_suppressed->Dump();
+  }
   self_->ClearException();
 }