Fix undefined behavior in closing a moved pipe

`pipe_next_read` is moved in the body of the loop, and not
re-initialized the last go around. However, we call
`pipe_next_read.close()` after the loop, which is undefined behavior (as
it's been moved).

Best case scenario, the compiler passed the address of our copy of the
struct to `exec_process_in_job` and beyond, it went out of scope there,
the value of `fd` was set to closed (minus one), and we explicitly call
`.close()` again, in which case it does nothing.

Worst case scenario, the compiler re-uses the storage for the now-moved
struct for something else and our call to `.close()` ends up closing
some other value of `fd` (valid or invalid) and things break.

Aside from the fact that we obviously don't need to close it since it's
not assigned for the last process in the job, it's a RAII object so we
don't have to worry about manually closing it in the first place.

src/exec.cpp

@@ -1093,7 +1093,6 @@ bool exec_job(parser_t &parser, const shared_ptr<job_t> &j, const io_chain_t &bl
         }
         procs_launched += 1;
     }
-    pipe_next_read.close();
 
     // If our pipeline was aborted before any process was successfully launched, then there is
     // nothing to reap, and we can perform an early return.