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
diff --git a/camera/device/1.0/default/CameraDevice.cpp b/camera/device/1.0/default/CameraDevice.cpp
index a03bbc8..2dd6094 100644
--- a/camera/device/1.0/default/CameraDevice.cpp
+++ b/camera/device/1.0/default/CameraDevice.cpp
@@ -397,9 +397,11 @@
     CameraDevice* device = mem->handle.mDevice;
     if (device == nullptr) {
         ALOGE("%s: camera HAL return memory for a null device!", __FUNCTION__);
+        return;
     }
     if (device->mDeviceCallback == nullptr) {
         ALOGE("%s: camera HAL return memory while camera is not opened!", __FUNCTION__);
+        return;
     }
     device->mDeviceCallback->unregisterMemory(mem->handle.mId);
     {