The SQL Server Native Client 11.0 and maybe other ODBC drivers report
NVARCHAR(MAX) columns as SQL_WVARCHAR with size 0. This causes too small a
buffer to be emalloc'd, likely causing a segfault in the following. As we don't
know the real size of the column data, we treat such colums as
SQL_WLONGVARCHAR.
The related bug #67437 suggests that some drivers report a size of ~4GB. It is
not certain that this is really the case (there might be some integer overflow
involved, and anyway, there has been no feedback), so we do not cater for this
now. However, it would not be hard to treat all sizes above a certain threshold
in a similar way, i.e. as SQL_WLONGVARCHAR.
