[evolution] Avoid crash when formatting broken message (part of bug #663221)

[Milan Crha <mcrha src gnome org>]

commit 46897aae3a6c68af4d3a96f4275cb0c8c50b5f2f
Author: Milan Crha <mcrha redhat com>
Date:   Mon Feb 27 13:44:28 2012 +0100

    Avoid crash when formatting broken message (part of bug #663221)

 mail/em-format-html.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/mail/em-format-html.c b/mail/em-format-html.c
index ae2782d..35fca48 100644
--- a/mail/em-format-html.c
+++ b/mail/em-format-html.c
@@ -1927,6 +1927,8 @@ efh_text_plain (EMFormat *emf,
 	flags = efh->text_html_flags;
 
 	dw = camel_medium_get_content ((CamelMedium *) part);
+	if (!dw)
+		return;
 
 	/* Check for RFC 2646 flowed text. */
 	if (camel_content_type_is(dw->mime_type, "text", "plain")