tests/sys/fs/fusefs/read.cc: fix build on powerpc64

There sig_atomic_t is shorter than void *.
As result, it cannot keep pointer.

Assigning to void * is actually safe for us in a signal handler.

Reviewed by: asomers
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Fixes: 4f917847c9037d
Differential revision: https://reviews.freebsd.org/D32064