openrc: avoid converting failed PTY reads to size_t

[pva]

openrc-run and rc_logger used read()'s return value as a size_t without checking for errors. Since read() returns an ssize_t, a failure return value -1 was converted to SIZE_MAX.

This path was normally hidden because the reader processes retained slave_tty, preventing the master from reporting a hangup. However, read() may still fail for other reasons, so handle its result explicitly.

With this path handled, it is also safe to close the unused slave_tty descriptors.