EP_Fptr ($48), Get file pointer via offset into file entry image

IN:
      A = EP_FPTR
    BHL = pointer to Eprom File Entry in card at slot
    CDE = offset into file image (24bit), 0 = first byte of file

OUT:
    Fc = 0, File Eprom available, File Entry available
         BHL = file pointer into File Entry

    Fc = 1,
         A = RC_Onf
         File Eprom was not found in slot, or File Entry not available

 Registers changed after return:
    ...CDE../IXIY same
    AFB...HL/.... different

; BHL points to File Entry definition

        ld      de,512                          ; CDE = file pointer to the 512th position of the file image
        ld      c,0
        ld      a,EP_Fptr
        oz      OS_Epr                          ; get ext address to file entry data

        ld      c,128
        ld      de,Buffer                       ; copy 128 bytes from file entry location into (DE)
        oz      OS_Bhl