json_serializer: handled corrupted files

When the `/var/lib/usr_mgr.conf` file is either empty or corrupted
JSON, the daemon will crash and not recover.  Handle this by catching
JSON load exceptions and deleting the corrupted file.

Fixes openbmc/phosphor-user-manager#19.

Tested: Added additional test cases to cover the corruption case and
update the test case to cover the non-throwing behavior.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I2be787771ea3d72af924615a6eee17cf2f393e9a
diff --git a/test/json_serializer_test.cpp b/test/json_serializer_test.cpp
index 50ac53f..4465918 100644
--- a/test/json_serializer_test.cpp
+++ b/test/json_serializer_test.cpp
@@ -108,8 +108,8 @@
     ofs.close();
 
     JsonSerializer s(test_file);
-    // nlohmann::json will throw a parse error for empty files or invalid JSON
-    EXPECT_THROW(s.load(), nlohmann::json::parse_error);
+    EXPECT_FALSE(s.load());
+    EXPECT_FALSE(std::filesystem::exists(test_file));
 }
 
 TEST_F(JsonSerializerTest, LoadGarbledJsonFile)
@@ -120,6 +120,6 @@
     ofs.close();
 
     JsonSerializer s(test_file);
-    // nlohmann::json will throw a parse error for incomplete JSON
-    EXPECT_THROW(s.load(), nlohmann::json::parse_error);
+    EXPECT_FALSE(s.load());
+    EXPECT_FALSE(std::filesystem::exists(test_file));
 }