[3.15] gh-151112: Fix crash in compiler_mod() when entering the current compilation unit fails (GH-151234) (#151350)

(cherry picked from commit 937d89c)

Co-authored-by: Stan Ulbrych <stan@python.org>

Author: miss-islington

Python/compile.c

@@ -893,12 +893,15 @@ compiler_mod(compiler *c, mod_ty mod)
 {
     PyCodeObject *co = NULL;
     int addNone = mod->kind != Expression_kind;
+    assert(c->u == NULL);
     if (compiler_codegen(c, mod) < 0) {
         goto finally;
     }
     co = _PyCompile_OptimizeAndAssemble(c, addNone);
 finally:
-    _PyCompile_ExitScope(c);
+    if (c->u != NULL) {
+        _PyCompile_ExitScope(c);
+    }
     return co;
 }