Fix edge cases found in audit

[dajiaohuang]

Summary

• Handle empty input safely when trimming strings.
• Bound embedded CRC filename scanning and avoid a duplicate torrent path allocation.
• Return cleanly when multi-hash context allocation fails.
• Reject oversized template files before appending beyond the configured limit.
• Use unsigned and bounded bit-mask shifts.
• Fixes Avoid underflow when trimming an empty string #313, Bound the backward scan for embedded CRC32 filenames #314, Avoid allocating the torrent output path twice #315, Return NULL when rhash_init_multi allocation fails #316, Stop reading a printf template after the configured size limit #317, and Use unsigned values for bit-mask shifts #318.

Validation

Not run, per the assigned no-tests/no-build scope.