Use source_locationt::nil() instead of local static_cast

Cleanup, no change in behaviour: just use the existing (and previously
already used in multiple places) static nil() member of source_locationt
instead of repeating its implementation in several places.

Author: tautschnig

src/cpp/cpp_name.h

@@ -73,7 +73,7 @@ class cpp_namet:public irept
   const source_locationt &source_location() const
   {
     if(get_sub().empty())
-      return static_cast<const source_locationt &>(get_nil_irep());
+      return source_locationt::nil();
     else
       return static_cast<const source_locationt &>(
         get_sub().front().find(ID_C_source_location));

src/goto-programs/goto_program.h

@@ -497,7 +497,7 @@ class goto_programt
 
     explicit instructiont(goto_program_instruction_typet __type)
       : code(static_cast<const codet &>(get_nil_irep())),
-        _source_location(static_cast<const source_locationt &>(get_nil_irep())),
+        _source_location(source_locationt::nil()),
         _type(__type),
         guard(true_exprt())
     {

src/util/expr.cpp

@@ -176,7 +176,7 @@ const source_locationt &exprt::find_source_location() const
       return op_l;
   }
 
-  return static_cast<const source_locationt &>(get_nil_irep());
+  return source_locationt::nil();
 }
 
 template <typename T>