expr evaluating to 0 are "false" so array_indexof returning 0 fails

There is a bug in my array_indexof implementation. If the value is at index zero, the expression is 'expr -1 + 1' which results in zero. Reading the man page for expr more closely, exit code is 1 if the expression results in 0, otherwise it is 0.

I am not sure of the most clever way to solve this, but this is my solution.