lib: fix possible off-by-one in stream_put_prefix()

lib: fix possible off-by-one in stream_put_prefix()

The STREAM_WRITEABLE() call only checks if there is space for the
prefix in the stream but does not account for the prefixlen. The
stream_putc() call reduces available space by 1 and we can end
copying one byte too much and with "endp" off by one if we are
near the buffer end.

Instead of moving the stream_putc() call before STREAM_WRITEABLE(),
we check before hand for the required space, and open-code it. This
avoids a function call and verifying again the stream buffer.

Signed-off-by: Jorge Boncompte [DTI2] <jorge@dti2.net>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>