parse-util: fix conversion from size_t on s390 (#3147)

parse-util: fix conversion from size_t on s390 (#3147)

On s390 size_t is an unsigned long, nor an unsigned int. They both are
of the same size and can be cast to each other safely, but the compiler
still seems unhappy about incompatible pointers.

Fixes: 7c2da2ca8