Fix Rare Bug with Wrong Memory Allocation/deallocation in Complex Key Cache Dictionaries with String Fields Which Leads to Infinite Memory Consumption (looks Like Memory Leak)

Fix rare bug with wrong memory allocation/deallocation in complex key cache dictionaries with string fields which leads to infinite memory consumption (looks like memory leak). Bug reproduces when string size was a power of two starting from eight (8, 16, 32, etc). #6447 (alesapin)