nss.c: Fixed compilation warnings (Take Two)

nss.c:702: warning: pointer targets in passing argument 3 of
'Curl_extract_certinfo' differ in signedness

nss.c:702: warning: pointer targets in passing argument 4 of
'Curl_extract_certinfo' differ in signedness

Made sure the cast was correctly "unsigned char *" to "char *" and not
"unsigned char *" to "unsigned char *".

lib/nss.c

@@ -696,10 +696,8 @@ static void display_conn_info(struct connectdata *conn, PRFileDesc *sock)
       }
       Curl_ssl_init_certinfo(conn->data, i);
       for(i = 0; cert; cert = cert2) {
-        Curl_extract_certinfo(conn, i++,
-                              (unsigned char *) cert->derCert.data,
-                              (unsigned char *) cert->derCert.data +
-                                                cert->derCert.len);
+        Curl_extract_certinfo(conn, i++, (char *)cert->derCert.data,
+                              (char *)cert->derCert.data + cert->derCert.len);
         if(cert->isRoot) {
           CERT_DestroyCertificate(cert);
           break;