https://bugs.gentoo.org/878395

Fix Python bindings build with SWIG >= 4.3 (multi-phase module init).

SWIG 4.3+ emits the user %init block into SWIG_mod_exec(PyObject *),
which returns int (a Py_mod_exec slot). The hand-written "return nullptr"
on the GLib-import failure path cannot convert to int, so the C++ wrapper
fails to compile under Python 3.13/3.14:

  classes_wrap.cpp: In function 'int SWIG_mod_exec(PyObject*)':
  error: cannot convert 'std::nullptr_t' to 'int' in return

Return -1 instead, the correct failure value for a Py_mod_exec slot.

--- a/bindings/python/sigrok/core/classes.i
+++ b/bindings/python/sigrok/core/classes.i
@@ -73,7 +73,7 @@
      */
     if (!GLib) {
         fprintf(stderr, "Import of gi.repository.GLib failed.\n");
-        return nullptr;
+        return -1;
     }
     import_array();
 %}