Fix signed/unsigned warnings for MSVC.

c0fe12cd broke the Windows build because of signed/unsigned compares.

Change-Id: Ie63246a60d51a8ccab17d4e2af8d155ed31b7b0a
diff --git a/crypto/x509/t_req.c b/crypto/x509/t_req.c
index 8ab7aa0..6ee949b 100644
--- a/crypto/x509/t_req.c
+++ b/crypto/x509/t_req.c
@@ -144,7 +144,7 @@
         goto err;
       }
     } else {
-      int i;
+      size_t i;
       for (i = 0; i < sk_X509_ATTRIBUTE_num(sk); i++) {
         X509_ATTRIBUTE *a = sk_X509_ATTRIBUTE_value(sk, i);
         ASN1_OBJECT *aobj = X509_ATTRIBUTE_get0_object(a);
@@ -205,7 +205,7 @@
     if (exts) {
       BIO_printf(bio, "%8sRequested Extensions:\n", "");
 
-      int i;
+      size_t i;
       for (i = 0; i < sk_X509_EXTENSION_num(exts); i++) {
         X509_EXTENSION *ex = sk_X509_EXTENSION_value(exts, i);
         if (BIO_printf(bio, "%12s", "") <= 0) {