buffer: Do not call explicit_bzero with null arguments

This allows compiling and testing with undefined sanitizer.

Signed-off-by: Nikos Mavrogiannopoulos <nmav@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

src/buffer.c

@@ -195,7 +195,9 @@ static void buffer_shift(ssh_buffer buffer){
 int ssh_buffer_reinit(struct ssh_buffer_struct *buffer)
 {
     buffer_verify(buffer);
+    if (buffer->used > 0) {
         explicit_bzero(buffer->data, buffer->used);
+    }
     buffer->used = 0;
     buffer->pos = 0;