[poppler] Decryption bugfix

[Dan Sheridan]

Hi,

PDF seems to be slightly confusing when it comes to the number of bits
used for the encryption key. In the standard, p93 says:

        Length integer (Optional; PDF 1.4; only if V is 2 or 3) The
        length
                of the encryption key, in bits. The value must be a
                multiple of 8, in the range 40 to 128. Default value:
                40.
        
But, when defining the actual algorithms, it says (p101):

        Set the encryption key to the first n bytes of the output from
        the final MD5 hash, where n is always 5 for revision 2 but, for
        revision 3 or greater, depends on the value of the encryption
        dictionary's Length entry.

Currently, poppler uses the Length entry to determine the length of the
key in revision 2 documents. The patch below fixes this and allows me to
open IEC standards documents in Evince.

	Dan.

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