MXRoom: Fix enableEncryptionWithAlgorithm that was calling success several times 
MXRoom: Fix enableEncryptionWithAlgorithm that was calling success several times 

eventBackListener was nil in the block
matrix-org/matrix-ios-sdk,matrix-org/matrix-ios-sdk,matrix-org/matrix-ios-sdk,matrix-org/matrix-ios-sdk,matrix-org/matrix-ios-sdk