In case of an unknown/invalid encoding, id3_parse_string() will
return NULL, but the return value wasn't checked resulting
in segfault in id3_ucs4_length().  This is the only place
the return value wasn't checked.

Patch taken from Debian:
https://sources.debian.org/patches/libid3tag/0.15.1b-14/11_unknown_encoding.dpatch/

CVE: CVE-2017-11550
Upstream-Status: Pending
Signed-off-by: Ross Burton <ross.burton@intel.com>

diff -urNad libid3tag-0.15.1b~/compat.gperf libid3tag-0.15.1b/compat.gperf
--- libid3tag-0.15.1b~/compat.gperf	2004-01-23 09:41:32.000000000 +0000
+++ libid3tag-0.15.1b/compat.gperf	2007-01-14 14:36:53.000000000 +0000
@@ -236,6 +236,10 @@
 
     encoding = id3_parse_uint(&data, 1);
     string   = id3_parse_string(&data, end - data, encoding, 0);
+    if (!string)
+    {
+	continue;
+    }
 
     if (id3_ucs4_length(string) < 4) {
       free(string);
diff -urNad libid3tag-0.15.1b~/parse.c libid3tag-0.15.1b/parse.c
--- libid3tag-0.15.1b~/parse.c	2004-01-23 09:41:32.000000000 +0000
+++ libid3tag-0.15.1b/parse.c	2007-01-14 14:37:34.000000000 +0000
@@ -165,6 +165,9 @@
   case ID3_FIELD_TEXTENCODING_UTF_8:
     ucs4 = id3_utf8_deserialize(ptr, length);
     break;
+  default:
+  	/* FIXME: Unknown encoding! Print warning? */
+	return NULL;
   }
 
   if (ucs4 && !full) {