Do not create buffer row for TDS7+

In TDS7+ buffer row contains all blobs while in TDS5 blobs are send after
the fixed and variable types. This cause the buffer to require lot of
space. Not using buffer but sending directly to the wire avoid these
huge allocations.
The code was also underestimating the buffer size causing buffer overflow.

Signed-off-by: Frediano Ziglio <freddy77@gmail.com>
Fix buffer overflow writing data from server to bcp file.

Calling dbconvert with values <0 for destination length could cause
buffer overflows. Compute output length properly to avoid these overflows.

Signed-off-by: Frediano Ziglio <freddy77@gmail.com>