Impacted versions: Odoo 11 on windows server 2012

Steps to reproduce: 
Create a new out invoice, select a product and set a negative amount for unit price.
Print the invoice.

Current behavior:
in PDF the char "-" is shown as "|" even if the total amount is correct (amount1 - amount2 = total)

Expected behavior:
the printed invoice should show the char "-"

Video/Screenshot link (optional):
![image](https://user-images.githubusercontent.com/37513316/42877323-8c712b2e-8a89-11e8-9690-27f00e08b6b2.png)

this is the negative item on odoo:
![image](https://user-images.githubusercontent.com/37513316/42877389-bf04eba2-8a89-11e8-91c0-e38059673b42.png)


openerp/addons/base/ir/ir_qweb.py
def get_command(self):
        try:
            if os.name == 'nt':
                lessc = find_in_path('lessc.cmd')
            else:
                lessc = find_in_path('lessc')
        except IOError:
            lessc = 'lessc'
        lesspath = get_resource_path('web', 'static', 'lib', 'bootstrap', 'less')
        return [lessc, '-', '--no-js', '--no-color', '--include-path=%s' % lesspath]
def rjsmin(script):
    """ Minify js with a clever regex.
    Taken from http://opensource.perlig.de/rjsmin
    Apache License, Version 2.0 """
    def subber(match):
        """ Substitution callback """
        groups = match.groups()
        return (
            groups[0] or
            groups[1] or
            groups[2] or
            groups[3] or
            (groups[4] and '\n') or