[FIX] stock_landed_costs: avoid useless multiple writes

Initialise quant_dict inside for loop as doing outside loops
renders to be a quadratic write of quant object due to the fact that
previous quants are kept each time a new valuation line is read.
In case of posting a Landed Cost record with a huge amount of cost_lines
and huge amount of stock_move having several quants would result in a
extra overhead.

Closes #9005