va: validate inputs in vaSetDriverName() before use

Add CHECK_DISPLAY(dpy) before CTX(dpy) to validate the display handle,
and add a NULL check for driver_name before calling strlen(). Without
these guards, passing NULL for either argument causes a null-pointer
dereference instead of returning a proper error status.

This aligns vaSetDriverName() with the validation pattern used by
other public APIs in va.c.

Signed-off-by: Carl.Zhang <carl.zhang@intel.com>

va/va.c

diff --git a/va/va.c b/va/va.c
index d87661a..15b3e52 100644
--- a/va/va.c
+++ b/va/va.c
@@ -627,9 +627,10 @@
     VADriverContextP ctx;
     VAStatus vaStatus = VA_STATUS_SUCCESS;
     char *override_driver_name = NULL;
+    CHECK_DISPLAY(dpy);
     ctx = CTX(dpy);
 
-    if (strlen(driver_name) == 0 || strlen(driver_name) >= 256) {
+    if (!driver_name || strlen(driver_name) == 0 || strlen(driver_name) >= 256) {
         vaStatus = VA_STATUS_ERROR_INVALID_PARAMETER;
         va_errorMessage(dpy, "vaSetDriverName returns %s\n",
                         vaErrorStr(vaStatus));