HPCC-12116 Do not allocate memory for 0 size in rtlMalloc

malloc(0) is undefined, but we would prefer NULL.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #6363 from richardkchapman/rtlMalloc-zero

HPCC-12116 Do not allocate memory for 0 size in rtlMalloc

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>