camera: fix two potential null pointer uses

Caught by the static analyzer:
hardware/interfaces/camera/device/1.0/default/CameraDevice.cpp:401:9:
warning: Called C++ object pointer is null
[clang-analyzer-core.CallAndMessage]

Bug: None
Test: TreeHugger
Change-Id: Ia6d0a2ab4b4d04cfc171629c7dbf60ead181625d