Fix segfault in gnc_gsettings_get_settings_obj when GSettings schemas are not installed

Add NULL guards before dereferencing return values from GLib calls to prevent
segmentation faults when GSettings schemas are missing (e.g., in library-only builds).

The crash occurred when g_settings_schema_source_get_default() returned NULL
and was passed directly to g_settings_schema_source_lookup(), causing a NULL
pointer dereference. The existing G_IS_SETTINGS check came too late.

Changes:
- Add NULL check for schema_source before using it
- Add NULL check for schema before calling g_settings_new_full()
- Add NULL guard in gnc_settings_dump_schema_paths() for consistency
- All callers already handle NULL returns via G_IS_SETTINGS checks

Fixes: https://bugs.gnucash.org/show_bug.cgi?id=799740

https://claude.ai/code/session_01Cdnp3XkAQ29hG1eCxUSAUw

libgnucash/app-utils/gnc-gsettings.cpp

@@ -105,7 +105,21 @@ static GSettings * gnc_gsettings_get_settings_obj (const gchar *schema_str)
     auto full_name_str = normalize_schema_name (schema_str);
     auto full_name = full_name_str.c_str();
     auto schema_source {g_settings_schema_source_get_default()};
+    if (!schema_source)
+    {
+        PWARN ("No GSettings schema source available; cannot access schema %s", full_name);
+        LEAVE("");
+        return nullptr;
+    }
+
     auto schema {g_settings_schema_source_lookup(schema_source, full_name, true)};
+    if (!schema)
+    {
+        PWARN ("GSettings schema %s not found", full_name);
+        LEAVE("");
+        return nullptr;
+    }
+
     auto gset = g_settings_new_full (schema, nullptr, nullptr);
     DEBUG ("Created gsettings object %p for schema %s", gset, full_name);
 
@@ -545,6 +559,12 @@ gnc_settings_dump_schema_paths (void)
     gchar **non_relocatable;
 
     auto schema_source {g_settings_schema_source_get_default()};
+    if (!schema_source)
+    {
+        PWARN ("No GSettings schema source available; cannot dump schema paths");
+        return;
+    }
+
     g_settings_schema_source_list_schemas (schema_source, true,
                                            &non_relocatable, nullptr);