buffer: Add NULL check for 'buffer' argument

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>
Reviewed-by: Anderson Toshiyuki Sasaki <ansasaki@redhat.com>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>

src/buffer.c

@@ -299,6 +299,10 @@ int ssh_buffer_reinit(struct ssh_buffer_struct *buffer)
  */
 int ssh_buffer_add_data(struct ssh_buffer_struct *buffer, const void *data, uint32_t len)
 {
+    if (buffer == NULL) {
+        return -1;
+    }
+
     buffer_verify(buffer);
 
     if (data == NULL) {