[PATCH] Ensure convert_mime_object does not return NULL

In case the message is an empty string charset_convert_string will
return NULL which then leads to problems later.
---
 macros/mime.sl | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/macros/mime.sl b/macros/mime.sl
index 24902cd..3d58547 100644
--- a/macros/mime.sl
+++ b/macros/mime.sl
@@ -348,6 +348,9 @@ private define format_type (type, width)
 private define convert_mime_object (obj)
 {
    variable str = obj.message;
+   if (str == "")
+     return str;
+
    if (obj.encoding == "base64")
      str = decode_base64_string (str);
    else if (obj.encoding == "quoted-printable")
-- 
2.1.4


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