fix integer overflow in pwg_scan_measurement

[tanjiroK-coder]

1. the digit loop in pwg_scan_measurement accumulates into an int, so a name with ten or more digits wraps: pwgMediaForPWG("custom_max_9999999999x9999999999mm") gives width and length of -727380068.
2. value * numer overflows on its own for much shorter names since numer is up to 100000 for metres: custom_max_1000000x1000000in gives -1754967296.

Both accumulators are long long now and the return saturates at INT_MAX, so valid names decode exactly as before. The size name is remote input, it comes in on media-supported/media-ready and reaches here via cupsCheckDestSupported and cupsGetDestMediaByName, where a negative width then defeats the custom-size range check. Turned up running -fsanitize=undefined over the media name parsers, which fires at pwg-media.c:1065 and :1086; the two testpwg cases fail on master and pass with the change.

Assisted-by: Claude Code:claude-opus-5