mod_mimeparse: Return NULL in bbs_mime_message_parse if mk_mime fails.

Do not return an "empty" mime container if mk_mime fails, since this
will lead to a NULL dereference later inside
bbs_mime_make_bodystructure. This could occur if analyzing invalid
maildir files, such as swap files, inside the maildir "cur" directory.
While resulting behavior will not be correct since swap files should
not be present (or should be ignored if present), this should at least
avoid a crash.

LBBS-146 #close

Author: InterLinked1

modules/mod_mimeparse.c

@@ -382,11 +382,22 @@ struct bbs_mime_message {
 
 struct bbs_mime_message *bbs_mime_message_parse(const char *filename)
 {
-	struct bbs_mime_message *mime = malloc(sizeof(*mime));
+	GMimeMessage *message;
+	struct bbs_mime_message *mime;
+
+	/* This is more likely to fail, so do it first before allocating the container */
+	message = mk_mime(filename);
+	if (!message) {
+		/* Return NULL if we failed here, since bbs_mime_make_bodystructure expects mime->message to be non-NULL */
+		return NULL;
+	}
+
+	mime = malloc(sizeof(*mime));
 	if (ALLOC_FAILURE(mime)) {
+		g_object_unref(message);
 		return NULL;
 	}
-	mime->message = mk_mime(filename);
+	mime->message = message;
 	return mime;
 }