Fix error in ce9d85ee.

I used a size_t out of habit, but |RSA_public_decrypt| is an old-style
function.

Change-Id: Ibd94d03743fe0099d61578ec15c19fa5333127db
diff --git a/crypto/evp/p_rsa.c b/crypto/evp/p_rsa.c
index 5f6db76..e613dac 100644
--- a/crypto/evp/p_rsa.c
+++ b/crypto/evp/p_rsa.c
@@ -278,8 +278,8 @@
   }
 
   if (rctx->md == NULL) {
-    const size_t ret = RSA_public_decrypt(sig_len, sig, rctx->tbuf,
-                                          ctx->pkey->pkey.rsa, rctx->pad_mode);
+    const int ret = RSA_public_decrypt(sig_len, sig, rctx->tbuf,
+                                       ctx->pkey->pkey.rsa, rctx->pad_mode);
     if (ret < 0) {
       return 0;
     }