Description:
------------
json_last_error() doesn't work properly in arrays/objects.  If an error occurs when encoding a property/key, and it isn't the LAST property/key, json_last_error() is reset to 0.

json_last_error() should report the last parsing error from the last json_encode() call AS AN ENTIRETY, not the last internal encoding step that takes place inside the function.

Test script:
---------------
$bad_utf8 = quoted_printable_decode('=B0');

json_encode($bad_utf8);
print "ERROR: " . json_last_error() . "\n";

$a = new stdclass;
$a->foo = quoted_printable_decode('=B0');
json_encode($a);
print "ERROR: " . json_last_error() . "\n";

$b = new stdclass;
$b->foo = $bad_utf8;
$b->bar = 1;
json_encode($b);
print "ERROR: " . json_last_error() . "\n";

$c = array(
    'foo' => $bad_utf8,
    'bar' => 1
);
json_encode($c);
print "ERROR: " . json_last_error() . "\n";


Expected result:
----------------
ERROR: 5
ERROR: 5
ERROR: 5
ERROR: 5

Actual result:
--------------
ERROR: 5
ERROR: 5
ERROR: 0
ERROR: 0