hush: fix handling of heredocs starting with empty lines

function                                             old     new   delta
parse_stream                                        2748    2759     +11

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/shell/hush_test/hush-heredoc/heredoc_empty3.right b/shell/hush_test/hush-heredoc/heredoc_empty3.right
new file mode 100644
index 0000000..0b54a9c
--- /dev/null
+++ b/shell/hush_test/hush-heredoc/heredoc_empty3.right
@@ -0,0 +1,2 @@
+
+Ok