MdtRawDataMonitoring: Fix the indexing logic in MdtRawDataMonAlg

While confirming the memory leak fix introduced in !42092 (merged) is working (it indeed does), I realized I made a practically transparent mistake while calculating the index of the newly introduced vector. The outer-loop's index should be multiplied with the size of the inner-loop not with its own size. Since both are equal, 4 in this case, this doesn't make a practical difference but better be fixed.

cc: @mbigliet