fusefs: fix an uninitialized memory access in fuse_vnop_deallocate

If the FUSE_GETATTR issued to query a file's size during
fuse_vnop_deallocate failed for any reason, then fuse_vnop_deallocate
would attempt to destroy an uninitialized fuse_dispatcher struct, with a
crash the likely result. This bug only affects FUSE file systems that
implement FUSE_FALLOCATE, and is unlikely to be seen on those that don't
disable attribute caching.

Reported by: Coverity Scan
CID: 1505308
MFC after: 2 weeks