[utils] fix buffer overflow in Pskc::SetSalt

[alexanderkent]

This commit addresses a potential buffer overflow vulnerability in the Pskc::SetSalt method.

Root Issue:
The Pskc::SetSalt method copies data into mSalt without sufficiently checking that the combined length of the data being copied (saltPrefix, aExtPanId, aNetworkName) doesn't exceed the size of mSalt. This leads to a stack buffer overflow, which may cause stack corruption, resulting in undefined behavior and potential security risks.

Fix:

1. Added a check to ensure that the total length of data to be copied into mSalt doesn't exceed its size.
2. Added a warning log if the network name is too long and needs to be truncated.
3. Utilized std::min to copy only as much of the network name as can fit into the remaining space in mSalt.

This ensures that no buffer overflow occurs, thereby preventing stack corruption and potential security vulnerabilities.

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[jwhui]

Thanks for this contribution! Just some minor code-style suggestions to resolve the pretty-check failure.