non-equi join errors when table on the outside is empty..

```r
require(data.table) # current devel, commit 2715
x <- data.table(a=1L, b=2L)
y <- x[0L]

y[x, on=.(a>a, b<b)]
# Error in bmerge(i, x, leftcols, rightcols, io, xo, roll, rollends, nomatch,  : 
#   Intrnal error: nqmaxgrpArg is not a positive length-1 integer vector
# In addition: Warning message:
# In max(nqgrp) : no non-missing arguments to max; returning -Inf
```