[FIX] stock: avoid deleted records

Steps to reproduce:
- Inventory -> Products -> Product -> Create a tracked serial product P
- Click on update quantity and fill in 2 serial numbers
- Overview -> Internal Transfers -> New Immediate transfer
- Move 2 units of P to another location and save
- Reduce the done quantity to 1

Issue:
It will raise an error warning us that a record doesn't exist or has
been deleted.

When decreasing the qty_done of the stock.move.live, since we're in an
immediate_transfer, it will also adjust the initial demand.
Meaning it will call the move's `write()` which in turn, if the initial
demand is changed, can be unreserved.
This results in the deletion of the current move.line, which is still
accessed after in the loop, raising the error.

Part-of: odoo#129009

addons/stock/models/stock_move.py

@@ -367,6 +367,9 @@ def _process_decrease(move, quantity):
                     continue
                 ml.qty_done -= qty_ml_dec
                 quantity -= move.product_uom._compute_quantity(qty_ml_dec, move.product_uom, round=False)
+                if not ml.exists():
+                    # If decreasing the move line qty_done to 0 let it to be unlinked (i.e. for immediate transfers)
+                    continue
                 # Unreserve
                 if (not move.picking_id.immediate_transfer and move.reserved_availability < move.product_uom_qty):
                     continue