Fix bug in ssl_write_supported_elliptic_curves_ext

Passing invalid curves to mbedtls_ssl_conf_curves caused a crash later
in ssl_write_supported_elliptic_curves_ext. #373
diff --git a/library/ssl_cli.c b/library/ssl_cli.c
index 09fc337..1d8b33f 100644
--- a/library/ssl_cli.c
+++ b/library/ssl_cli.c
@@ -269,6 +269,12 @@
     for( info = mbedtls_ecp_curve_list(); info->grp_id != MBEDTLS_ECP_DP_NONE; info++ )
     {
 #endif
+        if( info == NULL )
+        {
+            MBEDTLS_SSL_DEBUG_MSG( 1, ( "invalid curve in ssl configuration" ) );
+            return;
+        }
+
         elliptic_curve_len += 2;
     }
 
@@ -288,7 +294,6 @@
     for( info = mbedtls_ecp_curve_list(); info->grp_id != MBEDTLS_ECP_DP_NONE; info++ )
     {
 #endif
-
         elliptic_curve_list[elliptic_curve_len++] = info->tls_id >> 8;
         elliptic_curve_list[elliptic_curve_len++] = info->tls_id & 0xFF;
     }