windows-nat: Don't change current_event.dwThreadId in handle_output_debug_string()

Using the 'catch-signal' test from the testsuite, on x86_64 Cygwin:

    $ ./gdb testsuite/outputs/gdb.base/catch-signal/catch-signal.exe
    [...]
    (gdb) catch signal
    Catchpoint 1 (standard signals)
    (gdb) r
    [...]
    Catchpoint 1 (signal SIGHUP), main () at
    ../../../gdb/testsuite/gdb.base/catch-signal.c:40
    40        raise (SIGHUP);               /* second HUP */
    (gdb) c
    Continuing.
    main () at ../../../gdb/testsuite/gdb.base/catch-signal.c:40
    40        raise (SIGHUP);               /* second HUP */
    Failed to resume program execution (ContinueDebugEvent failed, error 87)
    (gdb)

This error occurs because when handle_output_debug_string processes a Cygwin
signal message, it re-writes current_event.dwThreadId to reflect the thread that
the signal will be delivered to, which can be different to the thread reporting
the signal.

Altering current_event.dwThreadId() will cause ContinueDebugEvent() to be
applied to the wrong thread and fail.

So, rather than re-writing the thread id in current_event, use the thread
id by returning it.

With this patch applied this test now yields the expected result:

    $ ./gdb testsuite/outputs/gdb.base/catch-signal/catch-signal.exe
    [...]
    (gdb) catch signal
    Catchpoint 1 (standard signals)
    (gdb) r
    [...]
    Catchpoint 1 (signal SIGHUP), main () at
    ../../../gdb/testsuite/gdb.base/catch-signal.c:40
    40        raise (SIGHUP);               /* second HUP */
    (gdb) c
    Continuing.
    Catchpoint 1 (signal SIGHUP), main () at
    ../../../gdb/testsuite/gdb.base/catch-signal.c:42
    42        raise (SIGHUP);               /* third HUP */
    (gdb)

gdb/ChangeLog:

2015-04-22  Jon Turney  <jon.turney@dronecode.org.uk>

	* windows-nat.c (handle_output_debug_string): Don't change
	current_event.dwThreadId.
	(get_windows_debug_event): Use thread_id, rather than relying on
	current_event.dwThreadId being changed.

1 files changed, 1 insertions, 2 deletions

diff --git a/gdb/windows-nat.c b/gdb/windows-nat.c
index 6942d64387..be5d7e814b 100644
--- a/gdb/windows-nat.c
+++ b/gdb/windows-nat.c
@@ -850,7 +850,6 @@ handle_output_debug_string (struct target_waitstatus *ourstatus)
 					 __COPY_CONTEXT_SIZE, &n)
 		   && n == __COPY_CONTEXT_SIZE)
 	    have_saved_context = 1;
-	  current_event.dwThreadId = retval;
 	}
     }
 #endif
@@ -1509,7 +1508,7 @@ get_windows_debug_event (struct target_ops *ops,
 				  thread_id);
       current_thread = th;
       if (!current_thread)
-	current_thread = thread_rec (current_event.dwThreadId, TRUE);
+	current_thread = thread_rec (thread_id, TRUE);
     }
 
 out: