After climbing the wall trying to find one corrupt frame in a huge
.avi I was compressing to MPEG (probably due to a buffer over-run when
the file-server went off and did its own thing for a bit...) I have
now modified readpic.c to give warnings but not crash if a dud frame
is encountered. In this case mpeg2enc now simply re-uses the previous frame to
keep things reasonably invisible.
