Make sure base URL for resolve_url is absolute

qutebrowser/browser/webkit/webelem.py

@@ -509,6 +509,9 @@ class WebElementWrapper(collections.abc.MutableMapping):
         Return:
             A QUrl with the absolute URL, or None.
         """
+        if baseurl.isRelative():
+            raise ValueError("Need an absolute base URL!")
+
         for attr in ['href', 'src']:
             if attr in self:
                 text = self[attr].strip()

tests/unit/browser/webkit/test_webelem.py

@@ -958,3 +958,9 @@ def test_resolve_url(attributes, expected):
     elem = get_webelem(attributes=attributes)
     baseurl = QUrl('http://www.example.com/')
     assert elem.resolve_url(baseurl) == expected
+
+
+def test_resolve_url_relative_base():
+    elem = get_webelem(attributes={'href': 'foo'})
+    with pytest.raises(ValueError):
+        elem.resolve_url(QUrl('base'))