avformat/rmdec: Fix EOF check in the stream loop in ivr_read_header()

Fixes: long running loop
Fixes: ivr-timeout-42468cb797f52f025fb329394702f5d4d64322d6

Found-by: Paul Ch <paulcher@icloud.com>
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>