Fix CVE-2018-20430:

https://gnunet.org/bugs/view.php?id=5493
https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2018-20430
https://security-tracker.debian.org/tracker/CVE-2018-20430

Patch copied from upstream source repository:

https://gnunet.org/git/libextractor.git/commit/?id=b405d707b36e0654900cba78e89f49779efea110

From b405d707b36e0654900cba78e89f49779efea110 Mon Sep 17 00:00:00 2001
From: Christian Grothoff <christian@grothoff.org>
Date: Thu, 20 Dec 2018 22:47:53 +0100
Subject: [PATCH] fix #5493 (out of bounds read)

---
 src/common/convert.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/src/common/convert.c b/src/common/convert.c
index c0edf21..2be2108 100644
--- a/src/common/convert.c
+++ b/src/common/convert.c
@@ -36,8 +36,8 @@
  *  string is returned.
  */
 char *
-EXTRACTOR_common_convert_to_utf8 (const char *input, 
-				  size_t len, 
+EXTRACTOR_common_convert_to_utf8 (const char *input,
+				  size_t len,
 				  const char *charset)
 {
 #if HAVE_ICONV
@@ -52,7 +52,7 @@ EXTRACTOR_common_convert_to_utf8 (const char *input,
   i = input;
   cd = iconv_open ("UTF-8", charset);
   if (cd == (iconv_t) - 1)
-    return strdup (i);
+    return strndup (i, len);
   if (len > 1024 * 1024)
     {
       iconv_close (cd);
@@ -67,11 +67,11 @@ EXTRACTOR_common_convert_to_utf8 (const char *input,
     }
   itmp = tmp;
   finSize = tmpSize;
-  if (iconv (cd, (char **) &input, &len, &itmp, &finSize) == SIZE_MAX)
+  if (iconv (cd, (char **) &input, &len, &itmp, &finSize) == ((size_t) -1))
     {
       iconv_close (cd);
       free (tmp);
-      return strdup (i);
+      return strndup (i, len);
     }
   ret = malloc (tmpSize - finSize + 1);
   if (ret == NULL)
-- 
2.20.1