routeQuery calls route_single_stmt, which requires the GWBUF to be
contiguous. Earlier it was made contiguous (if needed) in
route_single_stmt. However, since the process of making a GWBUF
contiguous causes the original buffer to be freed, this would lead
to a double free later in routeQuery that frees the passed buffer.
This is prevented now by making the buffer contiguous before calling
route_single_stmt.
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