Fixed bug #74103 and bug #75054

Directly fail unserialization when trying to acquire an r/R
reference to an UNDEF HT slot. Previously this left an UNDEF and
later deleted the index/key from the HT.

What actually caused the issue here is a combination of two
factors: First, the key deletion was performed using the hash API,
rather than the symtable API, such that the element was not actually
removed if it used an integral string key. Second, a subsequent
deletion operation, while collecting trailing UNDEF ranges, would
mark the element as available for reuse (leaving a corrupted HT
state with nNumOfElemnts > nNumUsed).

Fix this by failing early and dropping the deletion code.