[2.7] bpo-34457: Python/ast.c: Add missing NULL check to alias_for_import_name(). (GH-8852) (GH-8858)

Reported by Svace static analyzer.
(cherry picked from commit 28853a249b1d0c890b7e9ca345290bb8c1756446)

Co-authored-by: Alexey Izbyshev <izbyshev@ispras.ru>
diff --git a/Python/ast.c b/Python/ast.c
index 318c0bb..9460325 100644
--- a/Python/ast.c
+++ b/Python/ast.c
@@ -2520,6 +2520,8 @@
             break;
         case STAR:
             str = PyString_InternFromString("*");
+            if (!str)
+                return NULL;
             PyArena_AddPyObject(c->c_arena, str);
             return alias(str, NULL, c->c_arena);
         default: