libpldmresponder: Use ASSERT_TRUE() to prevent NULL pointer deref

EXPECT_TRUE() logs a failure of the condition but allows execution to
continue. From the gtest documentation:

> The majority of the macros listed below come as a pair with an EXPECT_
> variant and an ASSERT_ variant. Upon failure, EXPECT_ macros generate
> nonfatal failures and allow the current function to continue running,
> while ASSERT_ macros generate fatal failures and abort the current
> function.

https://google.github.io/googletest/reference/assertions.html

Fixes: 5bfb0dcbbd39 ("Refactor buildFRUTable and update to Association tree")
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Change-Id: I226fe8302acaef74be2ef88aaff8de7898a6e862
diff --git a/libpldmresponder/test/libpldmresponder_fru_test.cpp b/libpldmresponder/test/libpldmresponder_fru_test.cpp
index 6921ad9..89cc440 100644
--- a/libpldmresponder/test/libpldmresponder_fru_test.cpp
+++ b/libpldmresponder/test/libpldmresponder_fru_test.cpp
@@ -139,13 +139,13 @@
     EXPECT_TRUE(node != NULL);
 
     node = pldm_entity_association_tree_find(entityTree.get(), &chassisEntity);
-    EXPECT_TRUE(node != NULL);
+    ASSERT_TRUE(node != NULL);
     test_pldm_entity_node* test_node = (test_pldm_entity_node*)node;
     EXPECT_TRUE((test_node->parent).entity_type == systemEntity.entity_type);
 
     node = pldm_entity_association_tree_find(entityTree.get(),
                                              &motherboardEntity);
-    EXPECT_TRUE(node != NULL);
+    ASSERT_TRUE(node != NULL);
     test_node = (test_pldm_entity_node*)node;
     EXPECT_TRUE((test_node->parent).entity_type == chassisEntity.entity_type);