lang_modify with list object and unnamed NULL hangs R

[hongooi73]

library(rlang)
lang_modify(list(a=1), NULL)

Leaves R unresponsive; the only fix is to shut it down from the OS. This is on Windows 10 Pro 64-bit.

This is an edge case, but hanging R seems a bit unfriendly. It should probably throw an error.

[egnha]

@Hong-Revo Good catch. On Mac OS (El Capitan) and R 3.4.0, this segfaults.