MXCrypto: Fix code that went into a dead-end
MXCrypto: Fix code that went into a dead-end

This is the reason why some tests of MXCryptoTests randomly fail.

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