We did a code refacotr here
https://sourceware.org/ml/gdb-patches/2013-11/msg00063.html

>	(get_current_thread): New function, factored out from ...
>	(add_current_inferior_and_thread): ... this.  Adjust.
>
>@@ -3332,18 +3371,8 @@ add_current_inferior_and_thread (char *wait_status)
>
>   inferior_ptid = null_ptid;
>
>-  /* Now, if we have thread information, update inferior_ptid.  First
>-     if we have a stop reply handy, maybe it's a T stop reply with a
>-     "thread" register we can extract the current thread from.  If
>-     not, ask the remote which is the current thread, with qC.  The
>-     former method avoids a roundtrip.  Note we don't use
>-     remote_parse_stop_reply as that makes use of the target
>-     architecture, which we haven't yet fully determined at this
>-     point.  */
>-  if (wait_status != NULL)
>-    ptid = stop_reply_extract_thread (wait_status);
>-  if (ptid_equal (ptid, null_ptid))
>-    ptid = remote_current_thread (inferior_ptid);
>+  /* Now, if we have thread information, update inferior_ptid.  */
>+  ptid = get_current_thread (wait_status);

but after the refactor, local variable ptid is used without
initialisation.  However, before this change, ptid is initialised to
null_ptid.  This error can be found by valgrind too...

==3298==    at 0x6B99BA: ptid_equal (ptid.c:80)
==3298==    by 0x4C67FF: get_current_thread (remote.c:3484)
==3298==    by 0x4C6951: add_current_inferior_and_thread (remote.c:3511)
==3298==    by 0x4C762C: extended_remote_create_inferior (remote.c:8506)
==3298==    by 0x5A5312: run_command_1 (infcmd.c:606)
==3298==    by 0x68B4FB: execute_command (top.c:463)
==3298==    by 0x5C7214: command_handler (event-top.c:494)
==3298==    by 0x5C78A3: command_line_handler (event-top.c:692)
==3298==    by 0x6DEB57: rl_callback_read_char (callback.c:220)
==3298==    by 0x5C7278: rl_callback_read_char_wrapper (event-top.c:171)
==3298==    by 0x5C72C2: stdin_event_handler (event-top.c:432)
==3298==    by 0x5C6194: gdb_wait_for_event (event-loop.c:834)

This patch initialises local variable ptid to null in get_current_thread.
We don't need to initialise ptid in add_current_inferior_and_thread,
so this patch also removes the ptid initialisation.

gdb/ChangeLog.arm:
2016-03-29  Andre Vieira <andre.simoesdiasvieira@arm.com>
  Backport from mainline
  2015-07-17  Yao Qi  <yao.qi@linaro.org>

  * remote.c (get_current_thread): Initialise ptid to null_ptid.
  (add_current_inferior_and_thread): Don't initialise ptid.