open62541: fix crash in arrays when server disconnects

[dirk-zimoch]

1. When the server disconnects, readArray() is called with NULL instead of bptr. In that case don't try to erase the old content.
2. When an mbbo without user defined enum strings connects to an enum node, it should initialize choice strings and values from the server even if the current value on the server is none of the allowed enum values.
3. To avoid a race condition when using thousands of nodes, make sure initial read has completed before setting up monitored items.

[ralphlange]

Finally managed to run a test against my PLC setup - good to go.