shunt-manager: Fix potential race between install and uninstall

If the installation fails while a shunt is concurrently uninstalled,
the entry could already be destroyed when trying to remove and destroy
it after acquiring the lock again in `install()`.

This change handles the conflict the same way trap-manager does since
69cbe2ca3f ("trap-manager: Wait for install to finish before
uninstalling").

Fixes: 616ff9a236 ("shunt-manager: Remove stored entries if installation fails")

src/libcharon/sa/shunt_manager.c

@@ -377,6 +377,10 @@ METHOD(shunt_manager_t, uninstall, bool,
 	entry_t *entry, *found = NULL;
 
 	this->lock->write_lock(this->lock);
+	while (this->installing)
+	{
+		this->condvar->wait(this->condvar, this->lock);
+	}
 	enumerator = this->shunts->create_enumerator(this->shunts);
 	while (enumerator->enumerate(enumerator, &entry))
 	{