[flang] Merge pull request flang-compiler/f18#736 from flang-compiler/pmk-fix-expo

Do not emit a bogus exponent on zero (0.e-1)

Original-commit: flang-compiler/f18@ab19f2e9a7a9daa57ed1c1767cc182bf7fff98e6
Reviewed-on: https://github.com/flang-compiler/f18/pull/736