The code below increments pos in a loop while 0 != isdigit(next[pos]), but on the first iteration next[pos] is the same as *next and since the loop is placed inside if (0 != isdigit(*next)) it is guaranteed that at least one iteration of the loop and at least one pos++ will be executed. As a consequence if (0 == pos) block is logically dead.

/* process array index component */
if (0 != isdigit(*next))
{
	for (pos = 0; 0 != isdigit(next[pos]); pos++)
		;

	if (0 == pos)
		return zbx_jsonpath_error(*pnext);

	...