Fix return values for raptor_iostream_{hexa,}decimal_write

(raptor_iostream_decimal_write, raptor_iostream_hexadecimal_write):
Return non-0 if number of objects returned from
raptor_iostream_write_bytes() does not match expected length.

Fixes Issue #0000575
http://bugs.librdf.org/mantis/view.php?id=575
Code tidy; use nobj for raptor_iostream_write_bytes() return value.

Related to Issue #0000575 fix