interp: heredocs get consumed by the first command process executed

[fungaren]

POC:

while read -r FOO BAR; do
  echo $FOO $BAR >/dev/stderr
  cat /not_exist_file
done <<EOF
  AAA BBB
  CCC DDD
EOF

fang@debian:~$ gosh test.sh 
AAA BBB
cat: /not_exist_file: No such file or directory
fang@debian:~$ bash test.sh 
AAA BBB
cat: /not_exist_file: No such file or directory
CCC DDD
cat: /not_exist_file: No such file or directory

[mvdan]

Thanks for reporting; this is a pretty severe bug actually, and I'm surprised it had not been spotted yet.