Fix internal error on integer/char formatting of a null handle

[EylonKrause]

The integer and char format specifiers (%h %x %d %o %b %c) accept a class handle, a chandle, or the null literal — checkArgType() explicitly permits those types. In a constant context, though, formatArg formatted them via arg.convertToInt().integer(), and convertToInt() returns an invalid (bad) ConstantValue for a null handle, so integer() does std::get<SVInt> on the wrong variant and aborts with internal compiler error: std::get: wrong index for variant.

Repro

module m; localparam string S = $sformatf("%d", null); endmodule

Also crashes with %x / %o / %b / %h / %c, and with a chandle in a constant function:

module m;
  function automatic string f(); chandle c = null; return $sformatf("%x", c); endfunction
  localparam string S = f();
endmodule

Fix

Guard the conversion in a small helper: when convertToInt() produces no usable integer, format the null handle as its numeric value of zero (SVInt::Zero) instead of dereferencing the missing value. This mirrors the variant-aware fix in #1916 for the %u/%z specifiers and matches how a null handle formats numerically. (A class handle is already rejected earlier in a constant context, so the null literal and a chandle are the values that actually reach formatArg.)

Added a regression test in EvalTests.cpp; the full unit test suite passes (22972 assertions in 2487 test cases).

Disclosure: this contribution was authored with an AI coding assistant (Claude) and reviewed before submission.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.03%. Comparing base (a5ab852) to head (1b0a64b).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1944   +/-   ##
=======================================
  Coverage   96.03%   96.03%           
=======================================
  Files         247      247           
  Lines       56855    56860    +5     
=======================================
+ Hits        54599    54605    +6     
+ Misses       2256     2255    -1     

Files with missing lines	Coverage Δ
source/ast/SFormat.cpp	97.57% <100.00%> (+0.03%)	⬆️

... and 1 file with indirect coverage changes

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