switch-case conversion: ensure case selection has source location

The expression operand may be lacking a source location (as was seen
on larger projects). Restore a source location by using the one from the
case statement.

Author: tautschnig

src/goto-programs/goto_convert.cpp

@@ -382,6 +382,8 @@ void goto_convertt::convert_switch_case(
 
     exprt::operandst &case_op_dest=cases_entry->second->second;
     case_op_dest.push_back(code.case_op());
+    if(case_op_dest.back().source_location().is_nil())
+      case_op_dest.back().add_source_location() = code.source_location();
   }
 }