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

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

commit 646978d9adc5 ("vgdb: Handle EINTR and EAGAIN more
consistently") introduced another 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 when the child needs to do a chdir and that chdir fails. If
that happens the parent would have gotten the wrong error code.

(cherry picked from commit 747ca4eb5fed5dd58a14391a997bb9e658e3b1c8)