bpo-41215: Make assertion in the new parser more strict (GH-21364)

diff --git a/Parser/pegen.c b/Parser/pegen.c
index 42f9e0c..e2cbf8b 100644
--- a/Parser/pegen.c
+++ b/Parser/pegen.c
@@ -525,7 +525,7 @@
 static int
 _get_keyword_or_name_type(Parser *p, const char *name, int name_len)
 {
-    assert(name_len != 0);
+    assert(name_len > 0);
     if (name_len >= p->n_keyword_lists ||
         p->keywords[name_len] == NULL ||
         p->keywords[name_len]->type == -1) {