nvram: 🛡️ Sentinel: [security improvement] Replace unsafe sprintf with snprintf

[ManupaKDU]

🚨 Severity: Medium
💡 Vulnerability: Unsafe C function sprintf is used in multiple places within nvram.c to format strings into fixed-size buffers. This could lead to buffer overflows if the formatted data exceeds the buffer size, allowing memory corruption or arbitrary code execution.
🎯 Impact: A malicious actor could craft specific nvram values or headers to trigger an overflow, potentially leading to privilege escalation or denial of service on the device running OpenWrt.
🔧 Fix: Replaced all instances of sprintf with snprintf, ensuring that formatting never writes past the allocated memory buffer.
✅ Verification: Ran sh test_ci.sh to verify tests pass. Visually inspected the code diff to ensure all sprintf calls were changed to snprintf.

Signed-off-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

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PR created automatically by Jules for task 11729375659188088495 started by @manupawickramasinghe

[google-labs-jules]

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