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
"""Improve and remove sensitive information from sass/less compilator error messages"""
        error = stderr.split('Load paths')[0].replace('  Use --trace for backtrace.', '')
        if 'Cannot load compass' in error:
            error += "Maybe you should install the compass gem using this extra argument:\n\n" \
                     "    $ sudo gem install compass --pre\n"
        error += "This error occured while compiling the bundle '%s' containing:" % self.xmlid
        for asset in self.stylesheets:
            if isinstance(asset, PreprocessedCSS):
                error += '\n    - %s' % (asset.url if asset.url else '<inline sass>')
        return error
class WebAsset(object):