title: Error when i try to produce html report with gcovr, 'NoneType' object has no attribute 'startswith'
body: <p>I get the error </p>
  
  <pre><code>'NoneType' object has no attribute 'startswith'
  </code></pre>
  
  <p>when trying to produce the html report with gcovr (tool for estimating the coverage of tests)</p>
  
  <p>Any idea how should I fix it? For info, I am  on Ubuntu 14.04.  See the detail error message below:</p>
  
  <pre><code>parallels:try_gcovr$ gcovr -r .  --html
  Traceback (most recent call last):
    File "/usr/local/bin/gcovr", line 1970, in &lt;module&gt;
      print_html_report(covdata, options.html_details)
    File "/usr/local/bin/gcovr", line 1373, in print_html_report
      ttmp = os.path.abspath(options.output).split('.')
    File "/usr/lib/python2.7/posixpath.py", line 367, in abspath
      if not isabs(path):
    File "/usr/lib/python2.7/posixpath.py", line 61, in isabs
      return s.startswith('/')
  AttributeError: 'NoneType' object has no attribute 'startswith'
  </code></pre>