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

On Fri, 5 Apr 2024 08:15:20 -0400

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)

Here's how the problem happens:

1. The call lands in src/main/subassign.c, do_subassign2_dflt().

2. do_subassign2_dflt() calls SubassignTypeFix() to prepare the operand
for the assignment.

3. Since the assignment is "stretching", SubassignTypeFix() calls
EnlargeVector() to provide the space for the assignment.

The bug relies on `x` not being IS_GROWABLE(), which may explain 
why a plain x[[4]] <- list() sometimes doesn't fail.

The future assignment result `x` is now expression(a, b, c, NULL), and
the old `x` set to expression(NULL, NULL, NULL) by SET_VECTOR_ELT(newx,
i, VECTOR_ELT(x, i)); CLEAR_VECTOR_ELT(x, i); during EnlargeVector().

4. But then the assignment fails, raising the error back in
do_subassign2_dflt(), because the assignment kind is invalid: there is
no way to put data.frames into an expression vector. The new resized
`x` is lost, and the old overwritten `x` stays there.

Not sure what the right way to fix this is. It's desirable to avoid
shallow_duplicate(x) for the overwriting assignments, but then the
sub-assignment must either succeed or leave the operand untouched.
Is there a way to perform the type check before overwriting the operand?

Best regards,
Ivan