[Rd] (inappropriate) manipulation of expression objects causes segfault (PR#7326)

[deepayan at stat.wisc.edu]

> version
         _
platform i686-pc-linux-gnu
arch     i686
os       linux-gnu
system   i686, linux-gnu
status   Under development (unstable)
major    2
minor    1.0
year     2004
month    10
day      25
language R
>
> foo <- expression(alpha, beta, gamma)
> foo[2]
expression(beta)
> foo[2] <- NA
> foo ## or str(foo)
Segmentation fault

(Same behaviour in R 1.9.1)

'foo[[2]] <- NA' works fine, though.

Deepayan