Initialise jq.input_cb{,_data} to NULL in jq_init()

To avoid causing segmentation faults when  input/1  is called in a
jq_state on which  jq_set_input_cb()  has not been called; e.g. the one
used by  jq --run-tests.

That segfault could also by fixed in run_jq_tests() by calling:

  jq_set_input_cb(jq, NULL, NULL);

But I think it makes sense to just make jq_init() initialise those
values to NULL.

Ref: jqlang#2717 (comment)

src/execute.c

@@ -1012,6 +1012,9 @@ jq_state *jq_init(void) {
   jq->exit_code = jv_invalid();
   jq->error_message = jv_invalid();
 
+  jq->input_cb = NULL;
+  jq->input_cb_data = NULL;
+
   jq->err_cb = default_err_cb;
   jq->err_cb_data = stderr;
 

tests/jq.test

@@ -1814,3 +1814,10 @@ true
 tojson | fromjson
 {"a":nan}
 {"a":null}
+
+
+# calling input/1 in a test doesn't crash jq
+
+try input catch .
+null
+"break"