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            current_exception when no longer valid.
diff --git a/src/ex_eval.c b/src/ex_eval.c
index 17e0fdb..f209c5f 100644
--- a/src/ex_eval.c
+++ b/src/ex_eval.c
@@ -640,8 +640,11 @@
     void
 discard_current_exception(void)
 {
-    discard_exception(current_exception, FALSE);
-    current_exception = NULL;
+    if (current_exception != NULL)
+    {
+	discard_exception(current_exception, FALSE);
+	current_exception = NULL;
+    }
     did_throw = FALSE;
     need_rethrow = FALSE;
 }
@@ -1978,7 +1981,10 @@
 	 * 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;