app: fix pylint 'possible variable use before assignment' error

Fixes these (new) pylint errors:
~/work/flight_review/flight_review/app ~/work/flight_review/flight_review
************* Module three_d
tornado_handlers/three_d.py:133:45: E0606: Possibly using variable 'start_timestamp' before assignment (possibly-used-before-assignment)
************* Module serve
serve.py:149:39: E0606: Possibly using variable 'ulog_file' before assignment (possibly-used-before-assignment)

app/serve.py

@@ -100,6 +100,7 @@ def _fixup_deprecated_host_args(arguments):
 show_ulog_file = False
 show_3d_page = False
 show_pid_analysis_page = False
+ulog_file = ''
 if args.file is not None:
     ulog_file = os.path.abspath(args.file)
     show_ulog_file = True

app/tornado_handlers/three_d.py

@@ -119,6 +119,7 @@ def get(self, *args, **kwargs):
         # - altitude requires an offset (to match the GPS data)
         # - it's worse for some logs where the estimation is bad -> acro flights
         #   (-> add both: user-selectable between GPS & estimated trajectory?)
+        start_timestamp = 0
         for i in range(len(gps_pos.data['timestamp'])):
             t = gps_pos.data['timestamp'][i] + utc_offset
             utctimestamp = datetime.datetime.utcfromtimestamp(t/1.e6).replace(