dvbsub: Check for errors from system()

libavcodec/dvbsubdec.c:145:5: warning: ignoring return value of ‘system’, declared with attribute warn_unused_result [-Wunused-result]
libavcodec/dvbsubdec.c:148:5: warning: ignoring return value of ‘system’, declared with attribute warn_unused_result [-Wunused-result]
