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

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