rtmpproto: Fix limelight authentication with abbreviated app names

When streaming to limelight, the app name is either a full
"appname/subaccount" or "appname/_definst_". In the latter case,
the app name can be simplified into simply "appname", but the
authentication hashing assumes the /_definst_ still to be present.

Signed-off-by: Martin Storsjö <martin@martin.st>