[flang] Merge pull request flang-compiler/f18#834 from flang-compiler/ps-zerodo

Add a check for DO step expressions being zero

Original-commit: flang-compiler/f18@a5a55d5f3fb71109afd5d94ba3fdd0051778a19c
Reviewed-on: https://github.com/flang-compiler/f18/pull/834