Support packets with 0 length data.

SNPP time of day packet (APID 10) contains only headers with no
additional user data.

edosl0util/stream.py

@@ -5,13 +5,8 @@ import os
 from collections import defaultdict, deque, namedtuple
 
 from edosl0util import headers
-from edosl0util.headers import (
-    GROUP_CONTINUING,
-    GROUP_FIRST,
-    GROUP_LAST,
-    GROUP_STANDALONE,
-    PrimaryHeader,
-)
+from edosl0util.headers import (GROUP_CONTINUING, GROUP_FIRST, GROUP_LAST,
+                                GROUP_STANDALONE, PrimaryHeader)
 from edosl0util.timecode import dt_to_cds
 
 LOG = logging.getLogger(__name__)
@@ -99,10 +94,10 @@ class BasicStream(object):
         # data length includes h2size
         total_size = h1size + h1.data_length_minus1 + 1
         data_size = h1.data_length_minus1 + 1 - h2size
-        if self.with_data:
+        data = None
+        if self.with_data and data_size:
             data = self._read(data_size)
         else:
-            data = None
             self.file.seek(data_size, os.SEEK_CUR)
             self._offset += data_size
         return self.Tracker(h1, h2, total_size, offset, data)
@@ -227,8 +222,7 @@ class PacketStream(object):
         if len(self._seek_cache):
             return self._seek_cache.popleft()
         try:
-            # namedtuple('Tracker', ['h1', 'h2', 'size', 'offset', 'data'])
-            h1, h2, data_size, offset, data = self._stream.next()
+            h1, h2, data_size, offset, data = next(self._stream)
         except PacketTooShort as err:
             if self._fail_on_tooshort:
                 raise