clean up native NMR handle only if not null

[mtfriesen]

Description

Resolves #2529.

Testing

Tested locally using XDP stress test. I highly recommend that eBPF enable driver verifier with fault injection to catch these kinds of bugs. That's all XDP does to repro. I've filed a feature request to add this coverage to eBPF: #2531.

Documentation

N/A.

[codecov]

Codecov Report

Merging #2530 (737ebed) into main (b054f0f) will decrease coverage by 0.15%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##             main    #2530      +/-   ##
==========================================
- Coverage   84.18%   84.04%   -0.15%     
==========================================
  Files         157      157              
  Lines       29037    29038       +1     
==========================================
- Hits        24446    24404      -42     
- Misses       4591     4634      +43     

Impacted Files	Coverage Δ
libs/execution_context/ebpf_native.c	90.94% <71.42%> (-0.10%)	⬇️

... and 13 files with indirect coverage changes