requester: Enhance the PLDM requester code

- Free the instance id on error paths
- Make the conditional checks consistent
- Make the unit tests robust

Signed-off-by: Tom Joseph <rushtotom@gmail.com>
Change-Id: Ieb70da2820ab818c36f5bf874a602fb090659836
diff --git a/requester/request.hpp b/requester/request.hpp
index 4097e10..2d74147 100644
--- a/requester/request.hpp
+++ b/requester/request.hpp
@@ -58,7 +58,7 @@
     int start()
     {
         auto rc = send();
-        if (rc != PLDM_SUCCESS)
+        if (rc)
         {
             return rc;
         }