c23 migration: rename variable, nullptr now a keyword

Author: ppannuto

examples/tests/crash_dummy/main.c

@@ -2,12 +2,12 @@
 
 #include <libtock/interface/button.h>
 
-volatile int* nullptr = 0;
+volatile int* zeroptr = 0;
 
 static void button_callback(__attribute__ ((unused)) returncode_t ret,
                             __attribute__ ((unused)) int          btn_num,
                             __attribute__ ((unused)) bool         val) {
-  __attribute__ ((unused)) volatile int k = *nullptr;
+  __attribute__ ((unused)) volatile int k = *zeroptr;
 }
 
 int main(void) {