[geary] Fix ContentTypeTest.guess_type_from_buf occasionally failing.

[Michael Gratton <mjog src gnome org>]

commit 01a43bbe070ef9ab3b9590d8f66d5ebcdfc7facc
Author: Michael James Gratton <mike vee net>
Date:   Thu May 3 17:01:18 2018 +1000

    Fix ContentTypeTest.guess_type_from_buf occasionally failing.

 src/engine/mime/mime-content-type.vala |    6 +++---
 1 files changed, 3 insertions(+), 3 deletions(-)
---
diff --git a/src/engine/mime/mime-content-type.vala b/src/engine/mime/mime-content-type.vala
index 59d530b..8f12e55 100644
--- a/src/engine/mime/mime-content-type.vala
+++ b/src/engine/mime/mime-content-type.vala
@@ -79,9 +79,9 @@ public class Geary.Mime.ContentType : Geary.BaseObject {
             int max_len = 4096;
             // XXX determine actual max needed buffer size using
             // xdg_mime_get_max_buffer_extents?
-            uint8[] data = (max_len > buf.size)
-                ? buf.get_bytes()[0:max_len - 1].get_data()
-                : buf.get_uint8_array();
+            uint8[] data = (buf.size <= max_len)
+                ? buf.get_uint8_array()
+                : buf.get_bytes()[0:max_len].get_data();
 
             // XXX might just want to use xdgmime lib directly here to
             // avoid the intermediate glib_content_type step here?