[v10.2.x] Auth: Fix a panic during logout when OAuth provider is not set (#80221)

Auth: check that oauthProvider is not nil in Logout() func (#79271)

check that oauthProvider is not nil

(cherry picked from commit bff2ac3627)

Co-authored-by: Mihai Doarna <mihai.doarna@grafana.com>

pkg/api/login.go

@@ -260,7 +260,9 @@ func (hs *HTTPServer) Logout(c *contextmodel.ReqContext) {
 			}
 		}
 		oauthProvider := hs.SocialService.GetOAuthInfoProvider(strings.TrimPrefix(authInfo.AuthModule, "oauth_"))
-		oauthProviderSignoutRedirectUrl = oauthProvider.SignoutRedirectUrl
+		if oauthProvider != nil {
+			oauthProviderSignoutRedirectUrl = oauthProvider.SignoutRedirectUrl
+		}
 	}
 
 	hs.log.Debug("Logout Redirect url", "auth.SignoutRedirectUrl:", hs.Cfg.SignoutRedirectUrl)