C++: Initialize a couple const globals

C++: Initialize a couple const globals

In C++ mode, we get:

 src/gdb/gdbserver/tdesc.c:43:33: error: uninitialized const ‘default_description’ [-fpermissive]
  static const struct target_desc default_description;
				  ^
 In file included from src/gdb/gdbserver/tdesc.c:19:0:
 src/gdb/gdbserver/tdesc.h:26:8: note: ‘const struct target_desc’ has no user-provided default constructor
  struct target_desc
	 ^

gdb/ChangeLog:
2015-08-21  Pedro Alves  <palves@redhat.com>

	* frame.c (null_frame_id): Explicitly zero-initialize.

gdb/gdbserver/ChangeLog:
2015-08-21  Pedro Alves  <palves@redhat.com>

	* tdesc.c (default_description): Explicitly zero-initialize.