Initialise jq_state->debug_cb{,_data} to NULL

To make debug/0 not call an uninitialised function pointer when using
--run-tests or when using a jq_state on which jq_set_debug_cb() has not
been called.

src/execute.c

@@ -1015,6 +1015,9 @@ jq_state *jq_init(void) {
   jq->input_cb = NULL;
   jq->input_cb_data = NULL;
 
+  jq->debug_cb = NULL;
+  jq->debug_cb_data = NULL;
+
   jq->err_cb = default_err_cb;
   jq->err_cb_data = stderr;