json_lib: inconsistent OOM handling during parser initialisation

A common pattern in JSON SQL functions is to call mem_root_dynamic_array_init() with init_buffer = NULL and init_alloc = JSON_DEPTH_DEFAULT, followed by json_scan_start().

The former considers the array usable even if allocation fails and returns success with je.stack.buffer == NULL. However, json_scan_start() unconditionally writes to that buffer: *((int*)(je->stack.buffer))= val.

An allocation failure can therefore cause a NULL pointer dereference instead of a graceful error.

The same issue affects the mem_root_dynamic_array_init() + json_path_setup() combination.

Another problem is memset(array->buffer, 0, array->size_of_element*array->max_element), which is practically equivalent to memset(NULL, 0, 0).