binder: don't dereference NULL

Simply forming a reference to NULL results in undefined behavior
regardless of how the reference is used. Since `outPollFd` and
`errPollFd` are potentially NULL, we should pass them as pointers here.

Caught by clang's static analyzer:
> frameworks/native/libs/binder/UtilsHost.cpp:145:14: warning: Forming
reference to null pointer [clang-analyzer-core.NonNullParamChecker]
> frameworks/native/libs/binder/UtilsHost.cpp:147:14: warning: Forming
reference to null pointer [clang-analyzer-core.NonNullParamChecker]

Bug: None
Test: TreeHugger

Change-Id: Idf8c8291bde0ce0624085afd143096c357246673