libappfuse: use an explicit buffer size

This patch makes lookup_name use an explicit buffer size, rather than
"whatever the union happens to give me." I don't know if it's the
correct size, but having a 0-length array in this sort of configuration
seems fragile, and will be a compilation error when
https://android-review.googlesource.com/#/c/platform/bionic/+/449363/
goes in.

Bug: 12231437
Test: Builds with the mentioned patch applied to Bionic
Change-Id: I5793c906b7c2417e9a58aa907ef11c282027edb2