Ignore PLDM message if status objects not created

The OCC status objects are not created until after the host@0-on file
has been removed. This now delays when the objects are created.
This introduced a window where the host notified occ-control that the
OCCs were running (via PLDM) before the status objects were created
which led to a segfault.
This commit will verify the status object exists when getting the PLDM
callback.

Verified on Rainier

Change-Id: I72404d1b5d89c5259cc9f857c8087096644ef318
Signed-off-by: Chris Cain <cjcain@us.ibm.com>
diff --git a/occ_status.hpp b/occ_status.hpp
index e02aaea..77fb82d 100644
--- a/occ_status.hpp
+++ b/occ_status.hpp
@@ -36,7 +36,7 @@
 namespace sdbusRule = sdbusplus::bus::match::rules;
 
 // OCC status instance. Ex. for "occ0", the instance is 0
-using instanceID = int;
+using instanceID = unsigned int;
 
 // IPMI sensor ID for a given OCC instance
 using sensorID = uint8_t;