[valadoc] charts: Fix error handling if gvRenderData() returns an error

[Florian Brosch <flobrosch src gnome org>]

commit 0bc6256c00f81cc13a25245dd95ec7ef8672ab61
Author: Philip Withnall <philip withnall collabora co uk>
Date:   Wed May 13 18:40:59 2015 +0200

    charts: Fix error handling if gvRenderData() returns an error
    
    If libgvc was not built properly or is running against incorrectly versioned
    plugins, gvRenderData() can fail and return a null string. Ensure libvaladoc
    handles this correctly.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=711590

 src/libvaladoc/charts/chart.vala          |    5 +++--
 src/libvaladoc/html/htmlmarkupwriter.vala |    8 +++++---
 2 files changed, 8 insertions(+), 5 deletions(-)
---
diff --git a/src/libvaladoc/charts/chart.vala b/src/libvaladoc/charts/chart.vala
index 03dab30..a6307d7 100644
--- a/src/libvaladoc/charts/chart.vala
+++ b/src/libvaladoc/charts/chart.vala
@@ -52,13 +52,14 @@ public class Valadoc.Charts.Chart : Api.Visitor {
                context.render (graph, file_type, file);
        }
 
-       public uint8[] write_buffer (string file_type) {
+       public uint8[]? write_buffer (string file_type) {
                if (context == null) {
                        context = factory.create_context (graph);
                }
 
-               uint8[] data;
+               uint8[]? data;
 
+               /* This will return null in data if it fails. */
                context.render_data (graph, file_type, out data);
                return data;
        }
diff --git a/src/libvaladoc/html/htmlmarkupwriter.vala b/src/libvaladoc/html/htmlmarkupwriter.vala
index 113f0c6..15ed9ef 100644
--- a/src/libvaladoc/html/htmlmarkupwriter.vala
+++ b/src/libvaladoc/html/htmlmarkupwriter.vala
@@ -44,9 +44,11 @@ public class Valadoc.Html.MarkupWriter : Valadoc.MarkupWriter {
        }
 
        public MarkupWriter add_usemap (Charts.Chart chart) {
-               string buf = (string) chart.write_buffer ("cmapx");
-               raw_text ("\n");
-               raw_text (buf);
+               string? buf = (string?) chart.write_buffer ("cmapx");
+               if (buf != null) {
+                       raw_text ("\n");
+                       raw_text ((!) buf);
+               }
 
                return this;
        }