Free Extended Memory Block (Function 0Ah)

        ARGS:   AH = 0Ah
                DX = Handle to the allocated block which should be freed

        RETS:   AX = 0001h if the block is successfully freed, 0000h otherwise

        ERRS:   BL = 80h if the function is not implemented
                BL = 81h if a VDISK device is detected
                BL = A2h if the handle is invalid
                BL = ABh if the handle is locked

This function frees a block of extended memory which was previously
allocated using Function 09h (Allocate Extended Memory Block). Programs
which allocate extended memory should free their memory blocks before
exiting. When an extended memory buffer is freed, its handle and all
data stored in it become invalid and should not be accessed.