util/find-doc-nits: check podchecker() return value

util/find-doc-nits: check podchecker() return value

From the Pod::Checker manual:

> RETURN VALUE
>        podchecker returns the number of POD syntax errors found or
>        -1 if there were no POD commands at all found in the file.

Reviewed-by: David von Oheimb <david.von.oheimb@siemens.com>
(Merged from https://github.com/openssl/openssl/pull/13416)