Initialise jq_state->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 be 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: https://github.com/jqlang/jq/pull/2717#discussion_r1264338841

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

@@ -1818,3 +1818,10 @@ true
 tojson | fromjson
 {"a":nan}
 {"a":null}
+
+
+# calling input/1 in a test doesn't crash jq
+
+try input catch .
+null
+"break"