formatter: avoid crash on attribute instance in for-loop header

[EylonKrause]

Problem

verible-verilog-format aborts with a CHECK failure (SIGABRT, exit 134)
when an attribute instance (* ... *) appears in a for-loop header.
TreeUnwrapper::ReshapeTokenPartitions' kForSpec case assumes the header
always produces two semicolon-leading partitions:

CHECK(iter1 != children.end());
...
CHECK(iter2 != children.end());

When an attribute instance appears in the init/condition, the partition
structure differs and a semicolon-leading partition is missing, firing the
CHECK and aborting the process on otherwise valid input.

Minimal repro

module m; initial for(int i=0(* a *);;) ; endmodule
module m; initial for(int i=0(* a *);i<4;i++) x=i; endmodule

Before this change both abort:

F tree-unwrapper.cc:2996] Check failed: iter2 != children.end()
*** Check failure stack trace: ***
... SIGABRT (exit 134)

Fix

Replace the two unconditional CHECKs with graceful early-outs: when a
required semicolon-leading partition is not found, leave the partitions
unreshaped instead of aborting. Normal for-loops (both partitions present)
reshape exactly as before.

After the change the same inputs format without aborting, e.g.:

module m;
  initial for (int i = 0 (* a *); i < 4; i++) x = i;
endmodule

Testing

• bazel test -c opt //verible/verilog/formatting:tree-unwrapper_test //verible/verilog/formatting:formatter_test — both pass.
• Added a formatter regression test for the attribute-in-for-header input.
• Verified a normal for-loop (for (int i=0; i<4; i++)) formats unchanged.

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