[le audio] Fix crash when terminating big during streaming

There could be one case that BIG is teared down due to BT shut down.
We need to validate iso_impl_ before using it.

Bug: 291516832
Tag: #bug
Test: atest net_test_btm_iso
Test: manual test with BT off and confirm no crash
Change-Id: Id3612d0f5651f91d5f7e871c6c38624c072cd874

system/stack/btm/btm_iso.cc

@@ -112,7 +112,8 @@ void IsoManager::CreateBig(uint8_t big_id,
 }
 
 void IsoManager::TerminateBig(uint8_t big_id, uint8_t reason) {
-  pimpl_->iso_impl_->terminate_big(big_id, reason);
+  if (pimpl_->IsRunning())
+    pimpl_->iso_impl_->terminate_big(big_id, reason);
 }
 
 void IsoManager::HandleIsoData(void* p_msg) {