os: Users of path_converter don't handle fd == -1 properly

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2018-02-20.16:24:10.469>
labels = ['type-bug', '3.7']
title = "os: Users of path_converter don't handle fd == -1 properly"
updated_at = <Date 2018-02-20.16:28:20.371>
user = 'https://github.com/izbyshev'

bugs.python.org fields:

activity = <Date 2018-02-20.16:28:20.371>
actor = 'izbyshev'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2018-02-20.16:24:10.469>
creator = 'izbyshev'
dependencies = []
files = []
hgrepos = []
issue_num = 32887
keywords = []
message_count = 1.0
messages = ['312421']
nosy_count = 2.0
nosy_names = ['larry', 'izbyshev']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue32887'
versions = ['Python 3.6', 'Python 3.7']