Fix segfault with 'set print object on' + 'whatis <struct>' & co
    
    Compiling GDB with a recent GCC exposes a problem:
    
      ../../gdb/typeprint.c: In function 'void whatis_exp(const char*, int)':
      ../../gdb/typeprint.c:515:12: warning: 'val' may be used uninitialized in this function [-Wmaybe-uninitialized]
        real_type = value_rtti_type (val, &full, &top, &using_enc);
        ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    
    The warning is correct.  There are indeed code paths that use
    uninitialized 'val', leading to crashes.  Inside the
    value_rtti_indirect_type/value_rtti_type calls here in whatis_exp:
    
      if (opts.objectprint)
        {
          if (((TYPE_CODE (type) == TYPE_CODE_PTR) || TYPE_IS_REFERENCE (type))
    	  && (TYPE_CODE (TYPE_TARGET_TYPE (type)) == TYPE_CODE_STRUCT))
            real_type = value_rtti_indirect_type (val, &full, &top, &using_enc);
          else if (TYPE_CODE (type) == TYPE_CODE_STRUCT)
    	real_type = value_rtti_type (val, &full, &top, &using_enc);
        }
    
    We reach those calls above with "set print object on", and then with
    any of:
    
      (gdb) whatis struct some_structure_type
      (gdb) whatis struct some_structure_type *
      (gdb) whatis struct some_structure_type &
    
    because "whatis" with a type argument enters this branch:
    
          /* The behavior of "whatis" depends on whether the user
    	 expression names a type directly, or a language expression
    	 (including variable names).  If the former, then "whatis"
    	 strips one level of typedefs, only.  If an expression,
    	 "whatis" prints the type of the expression without stripping
    	 any typedef level.  "ptype" always strips all levels of
    	 typedefs.  */
          if (show == -1 && expr->elts[0].opcode == OP_TYPE)
    	{
    
    which does not initialize VAL.  Trying the above triggers crashes like
    this:
    
      (gdb) set print object on
      (gdb) whatis some_structure_type
    
      Thread 1 "gdb" received signal SIGSEGV, Segmentation fault.
      0x00000000005dda90 in check_typedef (type=0x6120736573756170) at src/gdb/gdbtypes.c:2388
      2388      int instance_flags = TYPE_INSTANCE_FLAGS (type);
      ...
    
    This is a regression caused by a recent-ish refactoring of the code on
    'whatis_exp', introduced by:
    
      commit c973d0aa4a2c737ab527ae44a617f1c357e07364
      Date:   Mon Aug 21 11:34:32 2017 +0100
    
          Fix type casts losing typedefs and reimplement "whatis" typedef stripping
    
    Fix this by setting VAL to NULL in the "whatis TYPE" case, and
    skipping fetching the dynamic type if there's no value to fetch it
    from.
    
    New tests included.
    
    gdb/ChangeLog:
    2018-01-22  Pedro Alves  <palves@redhat.com>
    	    Sergio Durigan Junior  <sergiodj@redhat.com>
    
    	* typeprint.c (whatis_exp): Initialize "val" in the "whatis type"
    	case.
    
    gdb/testsuite/ChangeLog:
    2018-01-22  Pedro Alves  <palves@redhat.com>
    	    Sergio Durigan Junior  <sergiodj@redhat.com>
    
    	* gdb.base/whatis.exp: Add tests for 'set print object on' +
    	'whatis <struct>' 'whatis <struct> *' and 'whatis <struct> &'.