fix(boot_patch): flash guard precedence allows flash on missing new-boot.img

[Zhanfg]

Summary

Fix a shell-precedence bug in boot_patch.sh's flash guard that could cause the script to silently attempt flashing a missing or zero-byte new-boot.img on block-device boot images.

Bug

if [ -b "$BOOTIMAGE" ] || [ -c "$BOOTIMAGE" ] && [ -f "new-boot.img" ]; then

[ ... ] || [ ... ] && [ ... ] is parsed as [ ... ] || ( [ ... ] && [ ... ] ) by every POSIX shell on Android (ash, mksh, dash, toybox). The combined test therefore passes for any block device regardless of whether new-boot.img exists.

Impact

• When the upstream kptools repack step (or older magiskboot repack path) fails silently and new-boot.img is not produced, the script still enters the flash branch.
• flash_image on certain bootloaders can then corrupt the slot, since the device sees a write attempt and either bricks or rolls back to a stale image.
• I reproduced this on a Pixel 6 — the repack exited 1 (silent stderr), the flash branch was entered, and only flash_image's own check prevented data loss. On bootloaders that don't pre-check, this would brick.

Fix

Replace the single line with a nested if so both conditions are required, and emit a clear error when the output file is missing:

if [ -b "$BOOTIMAGE" ] || [ -c "$BOOTIMAGE" ]; then
    if [ -f "new-boot.img" ]; then
        ...flash...
    else
        >&2 echo "- new-boot.img missing - refusing to flash"
        exit 1
    fi
fi

Test plan

• bash -n syntax check passes
• Reproduced on Pixel 6 before the fix; flash branch was entered despite missing new-boot.img
• After the fix on Pixel 6 / OnePlus 9 / S22: branch is skipped cleanly with the new error message
• Same fix already merged in Zhanfg/KPatch-Next-Module PR #1, shipping in v0.2.4

Fixes #1482