Fix a condition in isc_dnsstream_assembler_incoming()

Before calling isc_buffer_putmem(), there is a condition to check
that 'buf_size' is greater than 0. At this point 'buf_size' is
guaranteed to be greater than zero, so either the condition is
redundant, or 'unprocessed_size' should be checked instead, which
seems more logical, because calling isc_buffer_putmem() with
'unprocessed_size' being zero is not useful, although harmless.

lib/isc/include/isc/dnsstream.h

@@ -540,7 +540,7 @@ isc_dnsstream_assembler_incoming(isc_dnsstream_assembler_t *restrict dnsasm,
 				return;
 			}
 
-			if (buf_size > 0) {
+			if (unprocessed_size > 0) {
 				isc_buffer_putmem(dnsasm->current,
 						  unprocessed_buf,
 						  unprocessed_size);