bt: stop the sdp memory allocation craziness

Clang static analyzer reports a memory leak.  Actually, the allocated
memory escapes here:

        record->attribute_list[record->attributes].pair = data;

but clang is correct that the memory might leak if len is zero.  We
know it isn't; assert that it is the case.

The craziness doesn't end there.  The memory is freed by
bt_l2cap_sdp_close_ch:

       g_free(sdp->service_list[i].attribute_list->pair);

which actually should have been written like this:

       g_free(sdp->service_list[i].attribute_list[0].pair);

The attribute_list is sorted with qsort; but indeed the first
entry of attribute_list should point to "data" even after the qsort,
because the first record has id SDP_ATTR_RECORD_HANDLE, whose
numeric value is zero.

But hang on.  The qsort function is

    static int sdp_attributeid_compare(
                const struct sdp_service_attribute_s *a,
                const struct sdp_service_attribute_s *b)
    {
        return (int) b->attribute_id - a->attribute_id;
    }

but no one ever writes attribute_id.  So it only works if qsort is
stable, and who knows what else is broken, but we can fix it by
setting attribute_id in the while loop.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
