This fixes a use-after-free noticed by ASAN which would occur when a
dictwatcher was still active on a dictionary when the dictionary was
freed.
fun! MakeWatch()
let d = {'foo': 'bar'}
call dictwatcheradd(d, 'foo', function('...'))
endfun
Patch-by: oni-link
Closes#5930
These tests are essentially affirming a regression vs Vim. In Vim,
:echo system('cat - &', 'foo')
returns "foo", because Vim internally wraps the command with shell-specific
syntax to redirect the streams from /dev/null[1].
That can't work in Nvim because we use pipes directly (instead of temp files)
and don't wrap the command with shell-specific redirection syntax.
References #3529
References #5241
[1] http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_09_03_02
