Fix SetAuthIdCookie's IsAuthIdConnect function's issues with mixed authid types

Apologies for diff being a little wonky since I also moved the function in question to a more appropriate file in addition to altering it.
GetSteamXId is guaranteed to return a string if GetAuthString already did, so no need to check for NULL or \0.
Just a minor nit, but if it'd be possible for them to be invalid individually you wouldn't want to skip the iteration if one of them fails, but still check the other, so you might want to invert the conditions and return the index instead of continuing.