Check mprotect result

mprotect can theoretically fail, which could then let one exploit
a vulnerable codec if one exists on the device.

Bug: 31350239
Change-Id: I7b99c190619f0fb2eb93119596e6da0d2deb8ba5
(cherry picked from commit 866c800c0624bb13eee44973cc8a2ecd0012de6e)