patch 8.0.0705: crash when there is an error in a timer callback

Problem:    Crash when there is an error in a timer callback. (Aron Griffis,
            Ozaki Kiichi)
Solution:   Check did_throw before discarding an exception.  NULLify
            current_exception when no longer valid.

src/ex_cmds2.c

@@ -1235,7 +1235,7 @@ check_due_timer(void)
 	    if (called_emsg)
 	    {
 		++timer->tr_emsg_count;
-		if (!did_throw_save && current_exception != NULL)
+		if (!did_throw_save && did_throw && current_exception != NULL)
 		    discard_current_exception();
 	    }
 	    did_emsg = did_emsg_save;

src/ex_eval.c

@@ -639,9 +639,12 @@ discard_exception(except_T *excp, int was_finished)
  */
     void
 discard_current_exception(void)
+{
+    if (current_exception != NULL)
     {
 	discard_exception(current_exception, FALSE);
 	current_exception = NULL;
+    }
     did_throw = FALSE;
     need_rethrow = FALSE;
 }
@@ -1978,7 +1981,10 @@ enter_cleanup(cleanup_T *csp)
 	 * there is an extra instance for every call of do_cmdline(), anyway.
 	 */
 	if (did_throw || need_rethrow)
+	{
 	    csp->exception = current_exception;
+	    current_exception = NULL;
+	}
 	else
 	{
 	    csp->exception = NULL;

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    705,
 /**/
     704,
 /**/