fusefrontend: catch ReadAt integer overflow
fusefrontend: catch ReadAt integer overflow

Discovered by xfstests generic/564 .

Failure was:

generic/564	- output mismatch (see /opt/fuse-xfstests/results//generic/564.out.bad)
    --- tests/generic/564.out	2021-05-08 21:11:05.307395966 +0200
    +++ /opt/fuse-xfstests/results//generic/564.out.bad	2021-05-19 19:01:16.912888879 +0200
    @@ -31,7 +31,7 @@
     source range beyond 8TiB returns 0

     destination range beyond 8TiB returns EFBIG
    -copy_range: File too large
    +copy_range: Function not implemented

rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs