Check if client exists before referencing client_id
Check if client exists before referencing client_id

gras100/asks-oauthlib,jsfan/requests-oauthlib,sigmavirus24/requests-oauthlib,singingwolfboy/requests-oauthlib,dongguangming/requests-oauthlib,requests/requests-oauthlib,elafarge/requests-oauthlib,abhi931375/requests-oauthlib,lucidbard/requests-oauthlib