_pickle: Fix whichmodule() (#3358)

_PyUnicode_FromId() can return NULL: replace Py_INCREF() with
Py_XINCREF().

Fix coverity report: CID 1417269.
diff --git a/Modules/_pickle.c b/Modules/_pickle.c
index 2a3e739..2525536 100644
--- a/Modules/_pickle.c
+++ b/Modules/_pickle.c
@@ -1705,7 +1705,7 @@
 
     /* If no module is found, use __main__. */
     module_name = _PyUnicode_FromId(&PyId___main__);
-    Py_INCREF(module_name);
+    Py_XINCREF(module_name);
     return module_name;
 }