goto_programt::get_target: fix const-ness of operations

Return a const iterator in a const context to avoid providing a means to
modify instructions in a goto program despite being in a const context.

Author: tautschnig

src/goto-programs/goto_convert.cpp

@@ -243,7 +243,7 @@ void goto_convertt::optimize_guarded_gotos(goto_programt &dest)
 
   // mark the goto targets
   unsigned cnt = 0;
-  for(const auto &i : dest.instructions)
+  for(auto &i : dest.instructions)
     if(i.is_goto())
       i.get_target()->target_number = (++cnt);
 

src/goto-programs/goto_program.h

@@ -395,7 +395,15 @@ class goto_programt
 
     /// Returns the first (and only) successor for the usual case of a single
     /// target
-    targett get_target() const
+    const_targett get_target() const
+    {
+      PRECONDITION(targets.size() == 1);
+      return targets.front();
+    }
+
+    /// Returns the first (and only) successor for the usual case of a single
+    /// target
+    targett get_target()
     {
       PRECONDITION(targets.size()==1);
       return targets.front();