cameraservice: handle invalid native_handle from vndk impl

Cameraservice creates a H2BGraphicBufferProducer from a
native_handle_t. However, for native_handles that don't originate
from AImageReader_getWindowNativeHandle, the conversion returns
a nullptr. If this value is not checked, this results in cameraservice
crashing further down the stack.

This CL add a nullptr check and skips adding the IGBP if the
native_handle is malformed or invalid.

Bug: 309752167
Test: Verified by partner
Change-Id: I2e853f55d32f3cfe2fa51781f4eb87ee3de6607a