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/ash_test/ash-heredoc/heredoc_empty3.tests b/shell/ash_test/ash-heredoc/heredoc_empty3.tests
new file mode 100755
index 0000000..828c2dd
--- /dev/null
+++ b/shell/ash_test/ash-heredoc/heredoc_empty3.tests
@@ -0,0 +1,4 @@
+cat <<EOF
+
+Ok
+EOF