rtsp-auth: Fix NULL pointer dereference when handling an invalid basic Authorization header
When using the basic authentication scheme, we wouldn't validate that the authorization field of the credentials is not NULL and pass it on to g_hash_table_lookup(). g_str_hash() however is not NULL-safe and will dereference the NULL pointer and crash. A specially crafted (read: invalid) RTSP header can cause this to happen.
As a solution, check for the authorization to be not NULL before continuing processing it and if it is simply fail authentication.
This fixes CVE-2020-6095 and TALOS-2020-1018.
Discovered by Peter Wang of Cisco ASIG.