Parfait 1.1 reports:

Error: Null pointer dereference (CWE 476)
   Read from null pointer 'dv'
        at line 244 of Xext/xtest.c in function 'ProcXTestFakeInput'.
          Null pointer introduced at line 244.

which corresponds to:

    243         if (nev > 1 && !dev->valuator) {
    244             client->errorValue = dv->first_valuator;
    245             return BadValue;
    246         }

Unfortunately, dv is initialized to NULL before this:
    159     deviceValuator *dv = NULL;

and not set to an actual value until afterwards:
    250         for (n = 1; n < nev; n++) {
    251             dv = (deviceValuator *) (ev + n);

Should the reference at 244 be to the firstValuator variable set just
before that error check, or to another value?

If firstValuator works, then this would fix it, and prevent further invalid
uses:

diff --git a/Xext/xtest.c b/Xext/xtest.c
index 2abdc7f..6519b9c 100644
--- a/Xext/xtest.c
+++ b/Xext/xtest.c
@@ -156,7 +156,6 @@ ProcXTestFakeInput(ClientPtr client)
     DeviceIntPtr dev = NULL;
     WindowPtr root;
     Bool extension = FALSE;
-    deviceValuator *dv = NULL;
     ValuatorMask mask;
     int valuators[MAX_VALUATORS] = { 0 };
     int numValuators = 0;
@@ -241,14 +240,14 @@ ProcXTestFakeInput(ClientPtr client)
         }
 
         if (nev > 1 && !dev->valuator) {
-            client->errorValue = dv->first_valuator;
+            client->errorValue = firstValuator;
             return BadValue;
         }
 
         /* check validity of valuator events */
         base = firstValuator;
         for (n = 1; n < nev; n++) {
-            dv = (deviceValuator *) (ev + n);
+            deviceValuator *dv = (deviceValuator *) (ev + n);
             if (dv->type != DeviceValuator) {
                 client->errorValue = dv->type;
                 return BadValue;