I am getting weird crashes on `main` in `tests/sdc/alu_sub.ys` which I traced to a null `Wire*`
in `SdcObjects::constrained_ports`. The null `Wire*` is being set in the `SdcObjects`
constructor. I don't understand what's going on here, so I added this check to detect the
missing wire early ... and that made the crash go away. Compiler bug maybe? I have
`Debian clang version 19.1.7 (3+build5)`, default build configuration.
Anyway this code seems fine to have.
Fixes these build errors I'm getting locally with `tcl-devel-9.0.0-7.fc42.x86_64`.
I guess Tcl 9 broke this.
```
passes/cmds/sdc/sdc.cc:438:6: error: no matching function for call to 'Tcl_ListObjLength'
438 | if (Tcl_ListObjLength(interp, listObj, &listLength) == TCL_OK) {
| ^~~~~~~~~~~~~~~~~
/usr/include/tclDecls.h:1788:13: note: candidate function not viable: no known conversion from 'int *' to 'Tcl_Size *' (aka 'long *') for 3rd argument
1788 | EXTERN int Tcl_ListObjLength(Tcl_Interp *interp,
| ^
1789 | Tcl_Obj *listPtr, Tcl_Size *lengthPtr);
| ~~~~~~~~~~~~~~~~~~~
passes/cmds/sdc/sdc.cc:446:8: error: no matching function for call to 'Tcl_ListObjLength'
446 | if (Tcl_ListObjLength(interp, subListObj, &subListLength) == TCL_OK) {
| ^~~~~~~~~~~~~~~~~
/usr/include/tclDecls.h:1788:13: note: candidate function not viable: no known conversion from 'int *' to 'Tcl_Size *' (aka 'long *') for 3rd argument
1788 | EXTERN int Tcl_ListObjLength(Tcl_Interp *interp,
| ^
1789 | Tcl_Obj *listPtr, Tcl_Size *lengthPtr);
| ~~~~~~~~~~~~~~~~~~~
```
