[Rd] Possible ALTREP bug

[Gábor Csárdi]

I have found some weird SEXP corruption behavior with ALTREP, which
could be a bug. (Or I could be doing something wrong.)

I have an integer ALTREP vector that calls back to R from the Elt
method. When this vector is indexed in a lapply(), its first element
gets corrupted. Sometimes it's just a type change to logical, but
sometimes the corruption causes a crash.

I saw this on macOS from R 3.5.3 to 4.2.0. I created a small package
that demonstrates this: https://github.com/gaborcsardi/redfish

The R callback in this package calls `loadNamespace("Matrix")`, but
the same crash happens for other packages as well, and sometimes it
also happens if I don't load any packages at all. (But that example
was much more complicated, so I went with the package loading.)

It is somewhat random, and sometimes turning off the JIT avoids the
crash, but not always.

Hopefully I am just doing something wrong in the ALTREP code (see
https://github.com/gaborcsardi/redfish/blob/main/src/test.c), and it
is not actually a bug.

Thanks,
Gabor