Prevent crashes on bare Varbind arguments

[SoundGoof]

Summary

Fixes #5 by preventing interpreter crashes when a bare Varbind is passed to session methods that require a VarList.

Invalid arguments now produce direct Python exceptions instead of segmentation faults or secondary SystemError exceptions.

Root Cause

The native extension called PyObject_GetIter() but did not check whether iterator creation succeeded.

When a bare, non-iterable Varbind was supplied, iterator creation returned NULL and set a Python exception. Several methods then passed the null pointer to another API or decremented its reference count, causing a segmentation fault.

Other methods returned a value while an exception was still active, which Python reported as a SystemError.

Changes

• Check that iterator acquisition succeeds before using or releasing the iterator.
• Return NULL from native methods when a Python exception is active.
• Initialize cleanup references before use.
• Prevent double decrements on response-processing error paths.
• Apply the safety checks to:
  • get
  • getnext
  • set
  • walk
  • getbulk
• Add regression coverage for all five methods.

Behavior

Before this change:

• get, getnext, and set terminated the interpreter with SIGSEGV.
• walk and getbulk raised SystemError.

After this change:

• get, getnext, and set raise TypeError.
• walk and getbulk raise AttributeError.

Testing

• Built the native extension on Fedora 43 with Python 3.14 and Net-SNMP 5.9.4.
• Ran the focused five-method regression test.
• Verified each method in an isolated subprocess.
• Confirmed no method crashes the interpreter.
• Confirmed the native C file has no diagnostics.
• Confirmed the patch passes the whitespace check.