ssluse.c: fix compiler warning: conversion to 'int' from 'size_t'

Reported by Tatsuhiro Tsujikawa

http://curl.haxx.se/mail/lib-2012-06/0371.html

lib/ssluse.c

@@ -2786,7 +2786,7 @@ void Curl_ossl_random(struct SessionHandle *data, unsigned char *entropy,
                       size_t length)
 {
   Curl_ossl_seed(data); /* Initiate the seed if not already done */
-  RAND_bytes(entropy, length);
+  RAND_bytes(entropy, curlx_uztosi(length));
 }
 
 void Curl_ossl_md5sum(unsigned char *tmp, /* input */