Don't attempt to resolve permissions for invalid roles

This resolves #824.

Discord seems to have inconsistencies where a role can be deleted, but
there will still be a few users who still have it in their `role_ids`. I
was able to find this bug appearing in 11 members of a 10,000 member
guild, so it would make sense that this is relatively rare, and it's why
we hadn't noticed it previously.

Since our permission resolution code is implementation agnostic, it
operates on the user's RoleIds collection, which is what Discord sends
us directly, and is not vaidated against the member's guild.

In our permission resolution code, we make the assumption that Discord
will always be telling us the truth with regard to a member's
`role_ids`. This PR changes the behavior of permissions resolution to
instead verify that the guild was able to return a role before
attempting to resolve its permissions.

src/Discord.Net.Core/Utils/Permissions.cs

@@ -133,9 +133,10 @@ namespace Discord
                 ulong deniedPermissions = 0UL, allowedPermissions = 0UL;
                 foreach (var roleId in user.RoleIds)
                 {
-                    if (roleId != guild.EveryoneRole.Id)
+                    IRole role = null;
+                    if (roleId != guild.EveryoneRole.Id && (role = guild.GetRole(roleId)) != null)
                     {
-                        perms = channel.GetPermissionOverwrite(guild.GetRole(roleId));
+                        perms = channel.GetPermissionOverwrite(role);
                         if (perms != null)
                         {
                             allowedPermissions |= perms.Value.AllowValue;