hush: fix handling of empty heredoc EOF marker

function                                             old     new   delta
parse_stream                                        2609    2634     +25

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/shell/hush_test/hush-heredoc/heredoc_empty2.right b/shell/hush_test/hush-heredoc/heredoc_empty2.right
new file mode 100644
index 0000000..e32c6ea
--- /dev/null
+++ b/shell/hush_test/hush-heredoc/heredoc_empty2.right
@@ -0,0 +1,4 @@
+OK1
+Ok:0
+OK2
+Ok:0