Further libssl size_t-ify of reading

Writing still to be done

Reviewed-by: Rich Salz <rsalz@openssl.org>
diff --git a/ssl/statem/statem.c b/ssl/statem/statem.c
index 786eb24..901a3f2 100644
--- a/ssl/statem/statem.c
+++ b/ssl/statem/statem.c
@@ -490,12 +490,12 @@
 {
     OSSL_STATEM *st = &s->statem;
     int ret, mt;
-    unsigned long len = 0;
+    size_t len = 0;
     int (*transition) (SSL *s, int mt);
     PACKET pkt;
     MSG_PROCESS_RETURN(*process_message) (SSL *s, PACKET *pkt);
     WORK_STATE(*post_process_message) (SSL *s, WORK_STATE wst);
-    unsigned long (*max_message_size) (SSL *s);
+    size_t (*max_message_size) (SSL *s);
     void (*cb) (const SSL *ssl, int type, int val) = NULL;
 
     cb = get_callback(s);