fix: prevent panic on nil intermediate values during Set operations

[mfleader]

Fix panic on nil intermediate values during Set operations

Problem

The library panics with reflect: call of reflect.Value.Type on zero Value when setting values through JSON paths containing nil intermediate values. This violates RFC 6901 Section 7, which requires implementations to raise error conditions for unresolvable paths.

Reproduction

data := map[string]any{
    "level1": map[string]any{
        "level2": map[string]any{
            "level3": nil, // Causes panic
        },
    },
}

ptr, _ := jsonpointer.New("/level1/level2/level3/value")
ptr.Set(data, "test-value") // PANIC before fix, ERROR after fix

Solution

Added isNil() checks in two locations:

• setSingleImpl() (line 183): Before calling rValue.Type() on potentially zero reflect.Value
• set() traversal loop (line 294): Before reflection operations during path traversal

Returns descriptive errors like "cannot set field X on nil value" instead of panicking.

RFC 6901 Compliance

Per RFC 6901 Section 7: implementations must "raise an error condition" for unresolvable paths rather than crash.

Testing

Added comprehensive test cases covering direct nil traversal, multi-level paths, and path creation scenarios. All tests pass with proper error handling.

Impact

• No breaking changes - existing valid operations unchanged
• Crashes become recoverable errors
• Better compliance with JSON Pointer specification

[mfleader]

@fredbi, would you have time to review my changes?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.57%. Comparing base (76476c2) to head (0dd4df8).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #49      +/-   ##
==========================================
+ Coverage   69.97%   70.57%   +0.60%     
==========================================
  Files           2        2              
  Lines         343      350       +7     
==========================================
+ Hits          240      247       +7     
  Misses         78       78              
  Partials       25       25              

Flag	Coverage Δ
oldstable-macos-latest	70.57% <100.00%> (+0.60%)	⬆️
oldstable-ubuntu-latest	70.57% <100.00%> (+0.60%)	⬆️
oldstable-windows-latest	70.57% <100.00%> (+0.60%)	⬆️
stable-macos-latest	70.57% <100.00%> (+0.60%)	⬆️
stable-ubuntu-latest	70.57% <100.00%> (+0.60%)	⬆️
stable-windows-latest	70.57% <100.00%> (+0.60%)	⬆️

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[fredbi]

Excellent! Thank you