dragon_saver(): set_video_mode() return value ignored — on failure vid would alias the wrong (32KB text) video window and 64000-byte writes would overrun it

Summary

vid is captured unconditionally after the unchecked mode switch; every write trusts hardcoded 320x200. On failure va_window would describe the text window (32KB) and the 64000-byte writes run ~31KB past into the VGA BIOS shadow (MMIO, not kernel RAM - gpset still bounds pixel coords). Provably unreachable on this tree (same static mode table + flag validated by dragon_init; vga.c the only vidsw provider) - hardening for the generic vidsw contract. Check-and-fail-blank fix.