zookeeper.c:prepend_string func can dereference null ptr

All the callers of the function prepend_string make a call to prepend_string before checking that zhandle_t *zh is not null. At the top of prepend_string, zh is dereferenced without checking for a null ptr:

static char* prepend_string(zhandle_t zh, const char client_path) {
char *ret_str;
if (zh->chroot == NULL)
return (char *) client_path;

I propose fixing this by adding the check here in prepend_string:

static char* prepend_string(zhandle_t zh, const char client_path) {
char *ret_str;
if (zh==NULL || zh->chroot == NULL)
return (char *) client_path;