pad effect: fix a syntax checking corner case
pad effect: fix a syntax checking corner case

In certain cases when time-based and sample-based position
specifications were mixed, pad could reject valid parameters or
accept invalid ones, due to using a dummy sample rate. Postpone
the check to when the actual sample rate is known.

davel/sox,Motiejus/sox,pcqpcq/sox,mhartzel/sox_personal_fork,jacksonh/sox,CaptainHayashi/sox,mhartzel/sox_personal_fork,MageSlayer/sox,mhartzel/sox_personal_fork,pcqpcq/sox,uklauer/sox,MageSlayer/sox,jacksonh/sox,jacksonh/sox,uklauer/sox,Distrotech/sox,pcqpcq/sox,MageSlayer/sox,Distrotech/sox,CaptainHayashi/sox,cbagwell/sox,cbagwell/sox,CaptainHayashi/sox,Motiejus/sox,mhartzel/sox_personal_fork,uklauer/sox,MageSlayer/sox,pcqpcq/sox,MageSlayer/sox,cbagwell/sox,Distrotech/sox,davel/sox,jacksonh/sox,Distrotech/sox,cbagwell/sox,Motiejus/sox,CaptainHayashi/sox,Motiejus/sox,davel/sox,davel/sox,uklauer/sox