SVGHMI: Fixed typo on session manager unregister, leading to wrong count of sessions and then exceptions when creating more session than allowed in protocol options. Also added more safety check in protocol in case session would be missing.

          <xsd:attribute name="Command" type="xsd:string" use="optional" default="make -C %(buildpath)s all BEREMIZSRC=%(src)s BEREMIZCFLAGS=%(cflags)s MD5=%(md5)s USE_BEREMIZ=1 FROM_BEREMIZ=1"/>