vgdb.c (fork_and_exec_valgrind): Fix off-by-one error write

vgdb.c (fork_and_exec_valgrind): Fix off-by-one error write

commit 646978d9adc5 ("vgdb: Handle EINTR and EAGAIN more
consistently") introduced an off-by-one issue trying to write back the
error from child to parent.

Instead of +1 it should have been +written (which initially is zero).

This is in an "should never happen" path, so hopefully didn't really
cause issues. But if it did happen the parent would have gotten the
wrong error code.

(cherry picked from commit f4fe5faf3d0f45b3824bbb9070232682df52a582)