Description:
------------
The iconv_substr() function returns false instead of the expected substring.

I just tried UTF-16LE which works:

php -r "var_dump(iconv_substr(\"a\x00b\x00\", 0, 1, 'UTF-16LE'));"
string(2) "a\x00"

Also iconv_strlen() works as expected.

Test script:
---------------
var_dump(iconv_substr("\x00a\x00b", 0, 1, 'UTF-16BE'));

Expected result:
----------------
string(2) "\x00a"

Actual result:
--------------
bool(false)