After taking the lock on a lazy extension's state, check to see if it
was initialized while we were waiting for the lock.
Change-Id: I1cbd52e9d655eec6c9142c97689ae36f219a28f2
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/216898
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>