USB: usbtmc: add missing endpoint sanity check

USBTMC devices are required to have a bulk-in and a bulk-out endpoint,
but the driver failed to verify this, something which could lead to the
endpoint addresses being taken from uninitialised memory.

Make sure to zero all private data as part of allocation, and add the
missing endpoint sanity check.

Note that this also addresses a more recently introduced issue, where
the interrupt-in-presence flag would also be uninitialised whenever the
optional interrupt-in endpoint is not present. This in turn could lead
to an interrupt urb being allocated, initialised and submitted based on
uninitialised values.

Fixes: dbf3e7f6 ("Implement an ioctl to support the USMTMC-USB488 READ_STATUS_BYTE operation.")
Fixes: 5b775f67 ("USB: add USB test and measurement class driver")
Cc: stable <stable@vger.kernel.org>     # 2.6.28
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>