HPCC-8526 RtlDatasetBuilder wasn't updating self when it was reallocated

Calls to RtlDatasetBuilder::ensureCapacity() were increasing the size of the
target buffer, but not updating the value of self.  This could lead to
accessing invalid memory and other chaos.

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>