libnbaio: Don't call get_presentation_position for pre KitKat blobs

This fixes a crash with the HTC Tegra3 audio blob where
mStream->get_presentation_position is not NULL but pointing to a unknown
position in memory.

Change-Id: I0ff0ab064c1ac16a128bb3f24d4a473b3164f36d
Merge pull request #8 from CyanogenMod/cm-11.0

libnbaio: Don't call get_presentation_position for pre KitKat blobs