Fix memory ownership and allocation handling

[SoundGoof]

Summary

Fix memory ownership and allocation handling in netsnmp_walk().

The system-test workflow introduced in #19 exposed memory leaks under Valgrind and a heap-use-after-free under AddressSanitizer. This change:

• Removes access to a request PDU after Net-SNMP has freed it.
• Removes a duplicate OID allocation.
• Frees the final GETNEXT PDU when it will not be sent.
• Checks PDU and OID-array allocations.
• Safely cleans up partially completed allocations.
• Propagates allocation failures as Python MemoryError.

Testing

Executed the complete SNMP system-test suite in Podman:

• ASan/UBSan: 31 tests passed with no sanitizer errors.
• Valgrind: 31 tests passed with 0 errors.
• Valgrind reported 0 bytes definitely, indirectly, or possibly lost.
• GCC coverage job passed.