buffer.c: Fix null pointer dereference error

This issue was discovered by covscan tool.

Signed-off-by: Norbert Pocs <npocs@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

Author: Norbert Pocs

src/buffer.c

@@ -748,7 +748,8 @@ uint32_t ssh_buffer_get_u64(struct ssh_buffer_struct *buffer, uint64_t *data){
  */
 int ssh_buffer_validate_length(struct ssh_buffer_struct *buffer, size_t len)
 {
-    if (buffer->pos + len < len || buffer->pos + len > buffer->used) {
+    if (buffer == NULL || buffer->pos + len < len ||
+        buffer->pos + len > buffer->used) {
         return SSH_ERROR;
     }