empty dt, empty on, edge case join segfault

[jangorecki]

data.table()[data.table(a=1), on=character()]

 *** caught segfault ***
address 0x564227d970b8, cause 'memory not mapped'

Traceback:
 1: bmerge(i, x, leftcols, rightcols, roll, rollends, nomatch, mult,     ops, verbose = verbose)
 2: `[.data.table`(data.table(), data.table(a = 1), on = character())
 3: data.table()[data.table(a = 1), on = character()]

[tlapak]

Closed by #4272.

I can't actually test this right now but there's nothing about empty data.tables that suggests they should be able to get past the on checks.