bpo-34462: Add missing NULL check to _copy_raw_string() (GH-8863)

Reported by Svace static analyzer.
diff --git a/Modules/_xxsubinterpretersmodule.c b/Modules/_xxsubinterpretersmodule.c
index 6162c53..7b2cda2 100644
--- a/Modules/_xxsubinterpretersmodule.c
+++ b/Modules/_xxsubinterpretersmodule.c
@@ -15,7 +15,7 @@
         return NULL;
     }
     char *copied = PyMem_Malloc(strlen(str)+1);
-    if (str == NULL) {
+    if (copied == NULL) {
         PyErr_NoMemory();
         return NULL;
     }