Ensure we check i2d_X509 return val

The i2d_X509() function can return a negative value on error. Therefore
we should make sure we check it.

Issue reported by Yuan Jochen Kang.

Reviewed-by: Emilia Käsper <emilia@openssl.org>

ssl/ssl_cert.c

diff --git a/ssl/ssl_cert.c b/ssl/ssl_cert.c
index 04a4a36..a4bf76e 100644
--- a/ssl/ssl_cert.c
+++ b/ssl/ssl_cert.c
@@ -836,13 +836,18 @@
     unsigned char *p;
 
     n = i2d_X509(x, NULL);
-    if (!BUF_MEM_grow_clean(buf, (int)(n + (*l) + 3))) {
+    if (n < 0 || !BUF_MEM_grow_clean(buf, (int)(n + (*l) + 3))) {
         SSLerr(SSL_F_SSL_ADD_CERT_TO_BUF, ERR_R_BUF_LIB);
         return 0;
     }
     p = (unsigned char *)&(buf->data[*l]);
     l2n3(n, p);
-    i2d_X509(x, &p);
+    n = i2d_X509(x, &p);
+    if (n < 0) {
+        /* Shouldn't happen */
+        SSLerr(SSL_F_SSL_ADD_CERT_TO_BUF, ERR_R_BUF_LIB);
+        return 0;
+    }
     *l += n + 3;
 
     return 1;