[vte] widget: Fix advance_tail if tail is not in the stream

[Egmont Koblinger <egmontkob src gnome org>]

commit ecf7e0dafb95702a14c2e57e96b397600ccfc85b
Author: Egmont Koblinger <egmont gmail com>
Date:   Mon Nov 17 12:19:16 2014 +0100

    widget: Fix advance_tail if tail is not in the stream
    
    https://bugzilla.gnome.org/show_bug.cgi?id=738964

 src/ring.c           |    6 +++---
 src/vtestream-file.h |    2 ++
 2 files changed, 5 insertions(+), 3 deletions(-)
---
diff --git a/src/ring.c b/src/ring.c
index 267be8b..6eb3569 100644
--- a/src/ring.c
+++ b/src/ring.c
@@ -386,16 +386,16 @@ _vte_ring_discard_one_row (VteRing *ring)
        ring->start++;
        if (G_UNLIKELY (ring->start == ring->writable)) {
                _vte_ring_reset_streams (ring, ring->writable);
-       } else {
+       } else if (ring->start < ring->writable) {
                VteRowRecord record;
                _vte_stream_advance_tail (ring->row_stream, ring->start * sizeof (record));
                if (G_LIKELY (_vte_ring_read_row_record (ring, &record, ring->start))) {
                        _vte_stream_advance_tail (ring->text_stream, record.text_start_offset);
                        _vte_stream_advance_tail (ring->attr_stream, record.attr_start_offset);
                }
-       }
-       if (ring->start > ring->writable)
+       } else {
                ring->writable = ring->start;
+       }
 }
 
 static void
diff --git a/src/vtestream-file.h b/src/vtestream-file.h
index 47954ba..5c709b4 100644
--- a/src/vtestream-file.h
+++ b/src/vtestream-file.h
@@ -464,6 +464,8 @@ _vte_file_stream_advance_tail (VteStream *astream, gsize offset)
 {
        VteFileStream *stream = (VteFileStream *) astream;
 
+       g_assert(offset <= stream->head);
+
        if (offset >= stream->offset[0]) {
                stream->offset[1] = stream->offset[0];
                stream->offset[0] = stream->head;