Suppress spurious errors in PVR log.

When running in PVR mode get_iplayer.cgi sets `$SIG{CHLD} = 'IGNORE'` at line 311.
This means that `$?` returns -1 for all subsequent subprocess/pipe invocations, which in turn causes get_iplayer.cgi to emit spurious errors:
```
ERROR: Command failed to execute: No child processes
INFO: Command exit code 16777215
```
This patch suppresses those messages.