libfuse: null-terminate buffer in fuse_req_getgroups()

libfuse: null-terminate buffer in fuse_req_getgroups()

After reading the file /proc/$PID/task/$PID/status the buffer wasn't
terminated with a null character.  This could theoretically lead to buffer
overrun by the subsequent strstr() call.

Since the contents of the proc file are guaranteed to contain the pattern
that strstr is looking for, this doesn't happen in normal situations.

Add null termination for robustness.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>