[libxml2] Fix undefined behavior in UTF16LEToUTF8

[Nick Wellnhofer <nwellnhof src gnome org>]

commit 2f9382033e4c398dd1c9aae4d24fa9f649fbf23d
Author: Nick Wellnhofer <wellnhofer aevum de>
Date:   Mon Jun 15 15:45:47 2020 +0200

    Fix undefined behavior in UTF16LEToUTF8
    
    Don't perform arithmetic on null pointer.
    
    Found with libFuzzer and UBSan.

 encoding.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)
---
diff --git a/encoding.c b/encoding.c
index 8b6f349c..1a6386a4 100644
--- a/encoding.c
+++ b/encoding.c
@@ -496,13 +496,18 @@ UTF16LEToUTF8(unsigned char* out, int *outlen,
 {
     unsigned char* outstart = out;
     const unsigned char* processed = inb;
-    unsigned char* outend = out + *outlen;
+    unsigned char* outend;
     unsigned short* in = (unsigned short*) inb;
     unsigned short* inend;
     unsigned int c, d, inlen;
     unsigned char *tmp;
     int bits;
 
+    if (*outlen == 0) {
+        *inlenb = 0;
+        return(0);
+    }
+    outend = out + *outlen;
     if ((*inlenb % 2) == 1)
         (*inlenb)--;
     inlen = *inlenb / 2;