transfer.c: do not compute length of undefined hex buffer.

transfer.c: do not compute length of undefined hex buffer.

On non-ascii platforms, the chunked hex header was measured for char code
conversion length, even for chunked trailers that do not have an hex header.
In addition, the efective length is already known: use it.
Since the hex length can be zero, only convert if needed.

Reported by valgrind.