Fix undefined behavior (UB) from calling builtin_clz(0) am: c2b391b076 am: a738f586f1 am: 39516316e3 am: 015fd0f2a0

Original change: https://android-review.googlesource.com/c/platform/external/FP16/+/1592859

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: Ia930ccb8b15a76816080f8185a5881005006a5e7