The parfait 1.1 code analyzer reports:

Error: Buffer overrun
   Read outside array bounds (CWE 125): In array dereference of ev->valuators.data[i] with index 'i'
      Array size is 36 elements (of 8 bytes each), index >= 0 and index <= 39
        at line 1268 of Xi/exevents.c in function 'TouchCopyValuatorData'.

which corresponds to this code:

   1261 static void
   1262 TouchCopyValuatorData(DeviceEvent *ev, TouchPointInfoPtr ti)
   1263 {
   1264     int i;
   1265 
   1266     for (i = 0; i < sizeof(ev->valuators.mask) * 8; i++)
   1267         if (BitIsOn(ev->valuators.mask, i))
   1268             valuator_mask_set_double(ti->valuators, i, ev->valuators.data[i]);
   1269 }

In include/eventstr.h, DeviceEvent is defined with:
    struct {
        uint8_t mask[(MAX_VALUATORS + 7) / 8];/**< Valuator mask */
        uint8_t mode[(MAX_VALUATORS + 7) / 8];/**< Valuator mode (Abs or Rel)*/
        double data[MAX_VALUATORS];           /**< Valuator data */
    } valuators;

so while mask & mode round up to match the number of bytes in a bitmask,
data does not, thus leading to the 36 vs. 40 difference in array sizes.

Potential fixes include changing the DeviceEvent definition to round up
the number of data elements to the nearest multiple of 8 as well, or
changing the loop to limit the loop counter based on the size of the data
member instead of the mask member, or simply doing i < MAX_VALUATORS.

Of course, as long as the masks initialize the padding bytes to 0, this
shouldn't actually be hit in running code, just by code analyzers.