Do not cache an SDR that failed to build

init_sdr assigned self._sdr before initialize() ran, so a failure left the
half built object in the cache. The next call saw a non-None _sdr and handed
back that partial repository rather than trying again.

The visible symptom is a first call raising and the second appearing to
succeed. The real cost is on a bmc where the read fails once: the client
keeps the incomplete sdr for the life of the session and every later sensor
lookup answers from it without complaint.

confluent_server/aiohmi/ipmi/command.py

@@ -605,8 +605,12 @@ class Command(object):
             if hasattr(self._oem, 'init_sdr'):
                 self._sdr = await self._oem.init_sdr()
             else:
-                self._sdr = sdr.SDR(self, self._sdrcachedir)
-                await self._sdr.initialize()
+                # Assigned only once it is built. Assigning first meant a
+                # failed initialize left the half built object cached, and
+                # every later call returned that instead of retrying.
+                newsdr = sdr.SDR(self, self._sdrcachedir)
+                await newsdr.initialize()
+                self._sdr = newsdr
         return self._sdr
 
     async def get_event_constants(self):