Unlock guard before overflow panic

src/libstd/thread/mod.rs

@@ -550,6 +550,7 @@ impl ThreadId {
             // If we somehow use up all our bits, panic so that we're not
             // covering up subtle bugs of IDs being reused.
             if COUNTER == ::u64::MAX {
+                GUARD.unlock();
                 panic!("failed to generate unique thread ID: bitspace exhausted");
             }