HPCC-9442 Avoid ensureCapacity call on fixed size disk read

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #4495 from ghalliday/issue9442

HPCC-9442 Avoid ensureCapacity call on fixed size disk read

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>