[Rd] Bug in out-of-bounds assignment of list object to expression() vector

[June Choe]

There seems to be a bug in out-of-bounds assignment of list objects to an
expression() vector. Tested on release and devel. (Many thanks to folks
over at Mastodon for the help narrowing down this bug)

When assigning a list into an existing index, it correctly errors on
incompatible type, and the expression vector is unchanged:

```
x <- expression(a,b,c)
x[[3]] <- list() # Error
x
#> expression(a, b, c)
```

When assigning a list to an out of bounds index (ex: the next, n+1 index),
it errors the same but now changes the values of the vector to NULL:

```
x <- expression(a,b,c)
x[[4]] <- list() # Error
x
#> expression(NULL, NULL, NULL)
```

Curiously, this behavior disappears if a prior attempt is made at assigning
to the same index, using a different incompatible object that does not
share this bug (like a function):

```
x <- expression(a,b,c)
x[[4]] <- base::sum # Error
x[[4]] <- list() # Error
x
#> expression(a, b, c)
```

That "protection" persists until x[[4]] is evaluated, at which point the
bug can be produced again:

```
x[[4]] # Error
x[[4]] <- list() # Error
x
#> expression(NULL, NULL, NULL)
```

Note that `x` has remained a 3-length vector throughout.

Best,
June

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