lib/utf16.c: Clear errno if iconv() sets it to E2BIG

value_byte_runs() in xml/hivexml.c assumes that
hivex_value_struct_length() failed if it set errno to a nonzero value.
However, _hivex_recode() could internally set errno to nonzero on
success.  This resulted in hivexml failing and printing an error message
like the following:

	SYSTEM: Argument list too long