xf86drm: cap number of reported devices by drmGetDevice(2)

Do as the documentation says - when devices non NULL, cap the reported
devices to max_devices. Otherwise we risk out-of-bound access
for users of the API.

v2:
 - Fix this w/o breaking the API

v3:
 - Drop local variables, flip inverted conditional (Simon)

Issue: https://gitlab.freedesktop.org/mesa/drm/-/issues/56
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Simon Ser <contact@emersion.fr>

xf86drm.c

@@ -4272,6 +4272,10 @@ drm_public int drmGetDevices2(uint32_t flags, drmDevicePtr devices[],
     }
 
     closedir(sysdir);
+
+    if (devices != NULL)
+        return MIN2(device_count, max_devices);
+
     return device_count;
 }