Timeshift: fix crash on I/O error (CID#157)

(cherry picked from commit 636a434c)

modules/access_filter/timeshift.c

@@ -298,6 +298,7 @@ static void* Thread( vlc_object_t* p_this )
               block_Release( p_block );
               p_block = NULL;
             }
+            else
                 p_block->i_buffer = i_read;
         }