GHSA-cf4g-fcf8-3cr9: `pnet_packet` buffer overrun in `set_payload` setters

As indicated by this issue, a buffer overrun is possible in the set_payload setter of the various mutable “Packet” struct setters. The offending set_payload functions were defined within the struct impl blocks in earlier versions of the package, and later by the packet macro.

Fixed in the packet macro by this PR.