[utils] Fix TTML conversion

Tolerate invalid timestamps (closes #7909)

test/test_utils.py

@@ -661,8 +661,8 @@ ffmpeg version 2.4.4 Copyright (c) 2000-2014 the FFmpeg ...'''), '2.4.4')
             {'like_count': 190, 'dislike_count': 10}))
 
     def test_parse_dfxp_time_expr(self):
-        self.assertEqual(parse_dfxp_time_expr(None), 0.0)
-        self.assertEqual(parse_dfxp_time_expr(''), 0.0)
+        self.assertEqual(parse_dfxp_time_expr(None), None)
+        self.assertEqual(parse_dfxp_time_expr(''), None)
         self.assertEqual(parse_dfxp_time_expr('0.1'), 0.1)
         self.assertEqual(parse_dfxp_time_expr('0.1s'), 0.1)
         self.assertEqual(parse_dfxp_time_expr('00:00:01'), 1.0)
@@ -676,6 +676,9 @@ ffmpeg version 2.4.4 Copyright (c) 2000-2014 the FFmpeg ...'''), '2.4.4')
                     <p begin="0" end="1">The following line contains Chinese characters and special symbols</p>
                     <p begin="1" end="2">第二行<br/>♪♪</p>
                     <p begin="2" dur="1"><span>Third<br/>Line</span></p>
+                    <p begin="3" end="-1">Lines with invalid timestamps are ignored</p>
+                    <p begin="-1" end="-1">Ignore, two</p>
+                    <p begin="3" dur="-1">Ignored, three</p>
                 </div>
             </body>
             </tt>'''

youtube_dl/utils.py

@@ -1976,7 +1976,7 @@ def match_filter_func(filter_str):
 
 def parse_dfxp_time_expr(time_expr):
     if not time_expr:
-        return 0.0
+        return
 
     mobj = re.match(r'^(?P<time_offset>\d+(?:\.\d+)?)s?$', time_expr)
     if mobj:
@@ -2020,10 +2020,15 @@ def dfxp2srt(dfxp_data):
         raise ValueError('Invalid dfxp/TTML subtitle')
 
     for para, index in zip(paras, itertools.count(1)):
-        begin_time = parse_dfxp_time_expr(para.attrib['begin'])
+        begin_time = parse_dfxp_time_expr(para.attrib.get('begin'))
         end_time = parse_dfxp_time_expr(para.attrib.get('end'))
+        dur = parse_dfxp_time_expr(para.attrib.get('dur'))
+        if begin_time is None:
+            continue
         if not end_time:
-            end_time = begin_time + parse_dfxp_time_expr(para.attrib['dur'])
+            if not dur:
+                continue
+            end_time = begin_time + dur
         out.append('%d\n%s --> %s\n%s\n\n' % (
             index,
             srt_subtitles_timecode(begin_time),