reproducible seg fault in assignment by reference

[stevenbagley]

Reproducible seg fault in versions 1.9.4 and 1.9.5:

library(data.table)
# data.table 1.9.5  For help type: ?data.table
# *** NB: by=.EACHI is now explicit. See README to restore previous behaviour.
DT = data.table(x=rep(c("a","b","c"),each=3), y=c(1,3,6), v=1:9)
setkey(DT, x, y)
DT["a", z := NULL][]
# Error in `[.data.table`(DT, "a", `:=`(z, NULL)) : 
#  When deleting columns, i should not be provided
# In addition: Warning message:
# In `[.data.table`(DT, "a", `:=`(z, NULL)) :
#   Adding new column 'z' then assigning NULL (deleting it).
DT["a", z := 42L][]

# *** caught segfault ***
#address 0x300000003, cause 'memory not mapped'

#Traceback:
#1: `[.data.table`(DT, "a", `:=`(z, 42L))
#2: DT["a", `:=`(z, 42L)]

sessionInfo()
# R version 3.1.3 (2015-03-09)
# Platform: x86_64-apple-darwin13.4.0 (64-bit)
# Running under: OS X 10.10.2 (Yosemite)

# locale:
# [1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8

# attached base packages:
# [1] stats     graphics  grDevices utils     datasets  methods   base