No bounds check in df[i, :] with zero-column data frames

[nalimilan]

The second call should also throw a BoundsError:

julia> DataFrame(x=[])[1, :]
┌ Warning: Selecting a single row from a `DataFrame` will return a `DataFrameRow` in the future. To get a `DataFrame` use `df[row_ind:row_ind, :]`.
│   caller = top-level scope at none:0
└ @ Core none:0
ERROR: BoundsError: attempt to access 0-element Array{Any,1} at index [1]
Stacktrace:
 [1] getindex(::Array{Any,1}, ::Int64) at ./array.jl:739
 [2] getindex(::DataFrame, ::Int64, ::Colon) at /home/milan/.julia/dev/DataFrames/src/dataframe/dataframe.jl:288
 [3] top-level scope at none:0

julia> DataFrame()[1, :]
┌ Warning: Selecting a single row from a `DataFrame` will return a `DataFrameRow` in the future. To get a `DataFrame` use `df[row_ind:row_ind, :]`.
│   caller = top-level scope at none:0
└ @ Core none:0
0×0 DataFrame

[bkamins]

We have four problematic behaviors:

DataFrame()[1, :]
DataFrame()[1:2, :]
DataFrame()[1, Bool[]]
DataFrame()[1:2, Bool[]]

I will fix them after we merge #1603 not to mess up to much with it.

Fortunately SubDataFrame is OK.

[bkamins]

In #1603 I have fixed it in commit e87bf5a

[bkamins]

I think we can close it?