upsimage.cgi: adjust scaling for small ranges (single-digit)

[jimklimov]

Closes: #3469

Avoid lack of output when the range of a variable is small (e.g. 0-10) and division by zero occurred instead.

Practical testing needed (not done so far).

[github-actions]

A ZIP file with standard source tarball and another tarball with pre-built docs for commit 73fa2f4 is temporarily available: NUT-tarballs-PR-3496.zip.

[AppVeyorBot]

✅ Build nut 2.8.5.4852-master completed (commit 3471d3f670 by @jimklimov)

• artifacts
• A 7-zip archive with built NUT for Windows debug binaries is temporarily available: NUT-for-Windows-x86_64-SNAPSHOT-2.8.5.4852-master.7z

[AppVeyorBot]

✅ Build nut 2.8.5.4854-master completed (commit 6f7ae47734 by @jimklimov)

• artifacts
• A 7-zip archive with built NUT for Windows debug binaries is temporarily available: NUT-for-Windows-x86_64-SNAPSHOT-2.8.5.4854-master.7z

[AppVeyorBot]

✅ Build nut 2.8.5.4854-master completed (commit 6f7ae47734 by @jimklimov)

[mfrakka]

Tested on: Arch Linux, NUT 2.8.5, nutdrv_qx Q1, VulTech GS-3KVAS-RK (device reports battery.voltage as single 2 V cell)
Test command:
QUERY_STRING='host=vultech-3kva@192.168.150.1&display=battery.voltage&width=90' ./upsimage.cgi > out.png
echo "exit=$?" && xxd out.png | head -4

Result with stock 2.8.5: exit=136 (SIGFPE), empty output, Apache logs End of script output before headers.
Result with PR #3496 patch: exit=0, 798-byte output starting with Pragma: no-cache / Content-type: image/png followed by valid PNG signature (89 50 4e 47). Image renders correctly in browser.

Patch confirmed working. ✅