Don't advance our end pointer until we've checked we have enough buff…

…er left and have peeked ahead to see that a unicode escape is approaching.

Thanks @kivikakk for helping me track down the actual bug here!

Fixes: CVE-2017-16516
Origin: brianmario/yajl-ruby@a8ca8f4
Bug: lloyd/yajl#248
Bug-Debian: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1040036

src/cdogs/yajl/yajl_encode.c

@@ -139,8 +139,8 @@ void yajl_string_decode(yajl_buf buf, const unsigned char * str,
                     end+=3;
                     /* check if this is a surrogate */
                     if ((codepoint & 0xFC00) == 0xD800) {
-                        end++;
-                        if (str[end] == '\\' && str[end + 1] == 'u') {
+                        if (end + 2 < len && str[end + 1] == '\\' && str[end + 2] == 'u') {
+                            end++;
                             unsigned int surrogate = 0;
                             hexToDigit(&surrogate, str + end + 2);
                             codepoint =