Bug 1569405 - Avoid NS_ADDREF on nullptr principals by using NS_IF_ADDREF. r=smaug

Differential Revision: https://phabricator.services.mozilla.com/D172426

dom/base/nsContentPermissionHelper.cpp

@@ -691,7 +691,7 @@ nsContentPermissionRequestProxy::GetPrincipal(
     return NS_ERROR_FAILURE;
   }
 
-  NS_ADDREF(*aRequestingPrincipal = mParent->mPrincipal);
+  NS_IF_ADDREF(*aRequestingPrincipal = mParent->mPrincipal);
   return NS_OK;
 }
 
@@ -703,12 +703,7 @@ nsContentPermissionRequestProxy::GetTopLevelPrincipal(
     return NS_ERROR_FAILURE;
   }
 
-  if (!mParent->mTopLevelPrincipal) {
-    *aRequestingPrincipal = nullptr;
-    return NS_OK;
-  }
-
-  NS_ADDREF(*aRequestingPrincipal = mParent->mTopLevelPrincipal);
+  NS_IF_ADDREF(*aRequestingPrincipal = mParent->mTopLevelPrincipal);
   return NS_OK;
 }