Fix C compiler warning about implicit sign conversion (GH-4439)

Cython/Utility/ImportExport.c

@@ -768,7 +768,7 @@ static int __Pyx_MergeVtables(PyTypeObject *type) {
             base = base->tp_base;
         }
     }
-    base_vtables = (void**) malloc(sizeof(void*) * (base_depth + 1));
+    base_vtables = (void**) malloc(sizeof(void*) * (size_t)(base_depth + 1));
     base_vtables[0] = unknown;
     // Could do MRO resolution of individual methods in the future, assuming
     // compatible vtables, but for now simply require a common vtable base.