Do not allow to activate session multiple times
Do not allow to activate session multiple times

Otherwise it will add the session to the channel multiple times
which leads to an infinite loop during cleanup.

See https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=5576

Credit to oss-fuzz

jpfr/open62541,StalderT/open62541,JGrothoff/open62541,JGrothoff/open62541,open62541/open62541,jpfr/open62541,open62541/open62541,StalderT/open62541,JGrothoff/open62541,open62541/open62541,jpfr/open62541,StalderT/open62541,jpfr/open62541,open62541/open62541,StalderT/open62541,JGrothoff/open62541