s/auto_ptr/unique_ptr/

The old auto_ptr was used for simple RAII. None of the auto_ptr flaws
were apparent in that context, but given that that class is actively
deprecated and that compilers warn about its use, it's better to switch
to its drop-in replacement.

My reason for not using QScopedPointer is that the C++11 version works
well enough, Okular is being built in C++11 mode anyway, unique_ptr
works on the KF5-minimal-mandated-compilers according to their docs, and
therefore there's no point in using something with a different API
(QScopedPointer) just because its name begins with a Q.

REVIEW: 124027