hp5400: initialize options bit mask to avoid compiler warning and glitchy operation

This was picked up as a compiler warning in Ubuntu build and was a
real functional issue.

backend/hp5400_internal.c

@@ -165,6 +165,7 @@ GetSensors(THWParams * pHWParams, uint16_t *sensorMap)
    * Max 10 iterations for safety.
    *
    */
+  *sensorMap = 0;
   uint16_t thisSensorMap = 0;
   size_t iterCount = 10;
   do