log_services: Fix Severity Empty string found issue when running Redfish Service Validator

Symptom:
When running Redfish Service Validator, we found that Severity Empty string issue except first post code in PostCodesEntry.

*** /redfish/v1/Systems/system/LogServices/PostCodes/Entries/B1-1
Regetting resource from URI /redfish/v1/Systems/system/LogServices/PostCodes/Entries/B1-1
	 Type (#LogEntry.v1_4_0.LogEntry), GET SUCCESS (time: 0)
	 PASS

*** /redfish/v1/Systems/system/LogServices/PostCodes/Entries/B1-2
Regetting resource from URI /redfish/v1/Systems/system/LogServices/PostCodes/Entries/B1-2
	 Type (#LogEntry.v1_4_0.LogEntry), GET SUCCESS (time: 0)
Severity: Empty string found - Services should omit properties if not supported
Severity: Invalid Enum value '' found, expected ['OK', 'Warning', 'Critical']
	  FAIL...

Root cause:
In fillPostCodeEntry(), this statement "severity = message->severity" only be executed once.
And another statement {"Severity", std::move(severity)} in the end of this function will clear severity string to null after calling std::move() standard function.
Thus, only first post code Severity is 'OK', but the others are NULL.

Solution:
Move this statement "severity = message->severity" to for loop in fillPostCodeEntry() then all post codes will be recorded with correct severity string 'OK'.

Tested:
Passed the Redfish Service Validator and verified "Severity": "OK" in /redfish/v1/Systems/system/LogServices/PostCodes/Entries/B1-2
{
    "@odata.context": "/redfish/v1/$metadata#LogEntry.LogEntry",
    "@odata.id": "/redfish/v1/Systems/system/LogServices/PostCodes/Entries/B1-2",
    "@odata.type": "#LogEntry.v1_4_0.LogEntry",
    "Created": "2020-04-27T01:48:10+00:00",
    "EntryType": "Event",
    "Id": "B1-2",
    "Message": "Boot Count: 1: TS Offset: 0.0046; POST Code: 0x02",
    "MessageArgs": [
        "1",
        "0.0046",
        "0x02"
    ],
    "MessageId": "OpenBMC.0.1.BIOSPOSTCode",
    "Name": "POST Code Log Entry",
    "Severity": "OK"
}

Signed-off-by: Tim Lee <timlee660101@gmail.com>
Change-Id: Ibfccfa47cfe8e0f58e7f24650871edda5d248674
diff --git a/redfish-core/lib/log_services.hpp b/redfish-core/lib/log_services.hpp
index 2284e23..3f10725 100644
--- a/redfish-core/lib/log_services.hpp
+++ b/redfish-core/lib/log_services.hpp
@@ -2620,11 +2620,6 @@
     // Get the Message from the MessageRegistry
     const message_registries::Message *message =
         message_registries::getMessage("OpenBMC.0.1.BIOSPOSTCode");
-    std::string severity;
-    if (message != nullptr)
-    {
-        severity = message->severity;
-    }
 
     uint64_t currentCodeIndex = 0;
     nlohmann::json &logEntryArray = aResp->res.jsonValue["Members"];
@@ -2710,6 +2705,13 @@
             }
         }
 
+        // Get Severity template from message registry
+        std::string severity;
+        if (message != nullptr)
+        {
+            severity = message->severity;
+        }
+
         // add to AsyncResp
         logEntryArray.push_back({});
         nlohmann::json &bmcLogEntry = logEntryArray.back();