pola-run hangs when executable not found in PATH

CategoryFixed. Fixed in: 457 for PlashRelease1.18
Found in: 1.17

If the executable filename is to be looked up in PATH (i.e. doesn't contain a '/'), but it can't be found in PATH, resolve_executable_name() does not terminate. Instead, it consumes unbounded memory and causes a denial of service.

This affects pola-run (C implementation) and pola-shell.