Make sure that the interrupt interface has an endpoint before trying to
access its endpoint descriptors to avoid dereferencing a NULL pointer.

The driver binds to the interrupt interface with interface number 0, but
must not assume that this interface or its current alternate setting are
the first entries in the corresponding configuration arrays.

Fixes: b72458a8 ("[PATCH] USB: Eagle and ADI 930 usb adsl modem driver")
Cc: stable <stable@vger.kernel.org>     # 2.6.16
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20191210112601.3561-2-johan@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>