Avoid nil pointer exception when s.pending.response == nil
Avoid nil pointer exception when s.pending.response == nil

https://bugsnag.com/ninja-blocks-1/driver-go-zigbee/errors/54498f960d25313350facafb?event_id=54b3490e45323df73000c06e

Signed-off-by: Jon Seymour <13015cd9b4bd75fb137ba1a80810e07fcd27c10e@gmail.com>

ninjasphere/go-zigbee,ninjasphere/go-zigbee