Check num_children() == 0 in Column.from_column_view

[cwharris]

Description

This fixes a bug where Column.from_column_view is not verifying the existence of a string column's offsets child column prior to accessing it, resulting in a segmentation fault when passing a column_view from Column.view() to Column.from_column_view(...).

The issue can be reproduced with:

import cudf
from cudf.core.column.column import as_column
df = cudf.DataFrame({'a': cudf.Series([[]], dtype=cudf.core.dtypes.ListDtype('string'))})
s = df['a']
col = as_column(s)
col2 = cudf._lib.column.Column.back_and_forth(col)
print(col)
print(col2)

where back_and_forth is defined as:

    @staticmethod
    def back_and_forth(Column input_column):
        cdef column_view input_column_view = input_column.view()
        return Column.from_column_view(input_column_view, input_column)

I don't have the expertise to write the appropriate tests for this without introducing the back_and_forth function as an API, which seems undesirable.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[bdice]

This looks fine to me. We don't really have a way to test Cython-only methods at this time, as far as I am aware.

[vyasr]

This change looks correct to me, thanks! I agree with Bradley, there really isn't a great way to test this right now.