poppler: If the document is locked one can't use it

[tsdgeos]

without unlocking but since we obviously don't have
a password for the random data we just return

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[inferno-chromium]

@tsdgeos, can you please sign the cla and then we can merge.

[tsdgeos]

I don't plan on signing any CLA with Google, maybe we can pretend this is a trivial fix (which it actually is) and you found it also independently and you propose it in a different merge?

[titanous]

Done in #1437

[inferno-chromium]

Closing this one then.