filesrc: catch failure to seek back to zero after seek test
filesrc: catch failure to seek back to zero after seek test

This should never happen theoretically, but since a transient
failure would get us to silently read wrong data, it's worth
erroring out. And it silence this:

Coverity 206034

cablelabs/gstreamer,Lachann/gstreamer,Lachann/gstreamer,centricular/gstreamer,cfoch/gstreamer,surround-io/gstreamer,jpxiong/gstreamer,krichter722/gstreamer,surround-io/gstreamer,jpakkane/gstreamer,cfoch/gstreamer,centricular/gstreamer,Lachann/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,surround-io/gstreamer,mparis/gstreamer,centricular/gstreamer,mparis/gstreamer,StreamUtils/gstreamer,cfoch/gstreamer,krichter722/gstreamer,cfoch/gstreamer,jpxiong/gstreamer,shelsonjava/gstreamer,shelsonjava/gstreamer,jpakkane/gstreamer,Lachann/gstreamer,krichter722/gstreamer,jpxiong/gstreamer,krichter722/gstreamer,jpxiong/gstreamer,shelsonjava/gstreamer,surround-io/gstreamer,centricular/gstreamer,cablelabs/gstreamer,cablelabs/gstreamer,cablelabs/gstreamer,centricular/gstreamer,shelsonjava/gstreamer,jpakkane/gstreamer,Lachann/gstreamer,cfoch/gstreamer,shelsonjava/gstreamer,StreamUtils/gstreamer,jpakkane/gstreamer,cablelabs/gstreamer,jpxiong/gstreamer,mparis/gstreamer,StreamUtils/gstreamer,mparis/gstreamer,surround-io/gstreamer,mparis/gstreamer,jpakkane/gstreamer,krichter722/gstreamer