rtpgstdepay: Properly handle backward compat for event deserialization

rtpgstdepay: Properly handle backward compat for event deserialization

Actual code is checking for a NULL terminator and a ';' terminator,
for backward compat, in a chained way that cause all events being rejected.
The proper condition is to reject the events when terminator isn't
in ['\0', ';'] set.

https://bugzilla.gnome.org/show_bug.cgi?id=758151