Fix for PR mi/15863

Fix for PR mi/15863

If an MI client creates a varobj and attempts to update the root
/before/ the inferior is started, gdb will throw an internal error:

(gdb)
-var-create * - batch_flag
^done,name="var1",numchild="0",value="0",type="int",has_more="0"
(gdb)
-var-update var1
^done,changelist=[]
(gdb)
-var-update *
~"../../src/gdb/thread.c:628: internal-error: is_thread_state: Assertion `tp' failed.\nA problem internal to GDB has been detected,\nfurther debugging may prove unreliable.\nQuit this debugging session? "
~"(y or n) "

The function that handles the varobj update in the failing case,
mi_cmd_var_udpate_iter, checks if the thread/inferior is stopped before
attempting to update the varobj. It calls is_stopped (inferior_ptid)
which calls is_thread_state:

    tp = find_thread_ptid (ptid);
    gdb_assert (tp);

When there is no inferior, ptid is null_ptid, and find_thread_ptid (null_ptid)
returns NULL and the assertion is triggered.

This patch changes mi_cmd_var_update_iter to behave the same way
"-var-update var1" does: by calling the thread "stopped" if
there is no inferior (and thereby calling varobj_update_one).

ChangeLog
2014-06-16  Keith Seitz  <keiths@redhat.com>

	PR mi/15863
	* mi/mi-cmd-var.c (mi_cmd_var_update_iter): Do not attempt
	to update the varobj if inferior_ptid is null_ptid.

testsuite/ChangeLog
2014-06-16  Keith Seitz  <keiths@redhat.com>

	PR mi/15863
	* gdb.mi/mi-var-cmd.exp: Add test for -var-update before
	the inferior is started.