Remove parsing of non-existing TS_UD_SC_SEC1 fields

When RDP encryption method/level is NONE (CredSSP implies this case), TS_UD_SC_SEC1::encryptionLevel is the last present field. Subsequent fields aren't present in this case.

Change-Id: I44cfea3ed52fae1ed6c416aeb01c3cf6ff002a3c
Reviewed-on: https://code.wireshark.org/review/3545
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>