libbinder: only set sm stability if non-null

These mark*Stability functions actually abort for the nullptr case, but
when SM crashes, we'll get a null proxy here, and aborts everywhere
(instead of waiting in other processes).

Bug: one flake in binder_parcel_fuzzer weeks ago and more recently,
    discovered in some failure logs from kaleshsingh@
Test: manual (with crash in flattenBinder in SM)
Change-Id: I89787127d836d18d343161881dff6b156c2dc482