run-command: avoid `close(-1)` in `start_command()` error paths

When `start_command()` fails to set up a pipe partway through, it rolls back by closing the pipe ends it has already opened. For descriptors supplied by the caller rather than allocated locally, that rollback tested `if (cmd->in)` / `if (cmd->out)` before calling close(). The CHILD_PROCESS_INIT default of -1 ("no descriptor") is non-zero and so passes the test, meaning a caller that sets cmd->no_stdin or cmd->no_stdout without supplying a real fd ends up triggering close(-1) on the error path. The stdin-pipe failure branch a few lines above already uses the right idiom, `if (cmd->out > 0)`, which rejects both the -1 sentinel and 0 (the parent's own standard streams). Apply it to the three remaining rollback sites. Reported by Coverity as CID 1049722 ("Argument cannot be negative"). Assisted-by: Opus 4.7 Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>