INT 21 - DOS 2+ - FREE MEMORY
        AH = 49h
        ES = segment of block to free
Return: CF clear if successful
        CF set on error
            AX = error code (07h,09h) (see AH=59h)
Notes:  apparently never returns an error 07h, despite official docs; DOS 3.30
          code contains only an error 09h exit
        DOS 2.1-5.0 does not coalesce adjacent free blocks when a block is
          freed, only when a block is allocated or resized
SeeAlso: AH=4Ah