Ben Gamari authored May 13, 2018 and Ben Gamari committed May 13, 2018

Previously `showEFloat (Just 0) pi ""` would produce `3.0e0`. Of
course, this
blatantly disrespects the user's request to print with zero digits of
precision.
Fix this.

This is tested by base's `num008` testcase.

Test Plan: Validate

Reviewers: hvr

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #15115

Differential Revision: https://phabricator.haskell.org/D4665