refactor: replace calloc with g_new0 for struct allocations

Migrate structure allocations from calloc to glibs g_new0 macro to
improve type safety and memory robustness.

* Type Safety: The macro takes the type name directly, ensuring the
  allocated size always matches the pointer type.
* Static Analysis: It guarantees a non-NULL return by aborting on
  failure, which silences -fanalyzer warnings regarding potential NULL
  pointer dereferences.
* Readability: Removes redundant sizeof() calls and is the glib way

src/xmpp/form.c

@@ -74,7 +74,7 @@ _is_valid_form_element(xmpp_stanza_t* stanza)
 static DataForm*
 _form_new(void)
 {
-    DataForm* form = calloc(1, sizeof(DataForm));
+    DataForm* form = g_new0(DataForm, 1);
 
     return form;
 }
@@ -82,7 +82,7 @@ _form_new(void)
 static FormField*
 _field_new(void)
 {
-    FormField* field = calloc(1, sizeof(FormField));
+    FormField* field = g_new0(FormField, 1);
 
     return field;
 }