`--posix -e 'a\bar'` causes an error

[cakebaker]

Another issue found with the fuzzer:

$ printf "foo" | cargo run -q sed --posix -e 'a\bar'
sed: <script argument 1>:1:3: error: extra characters after \ at the end of `a' command
$ printf "foo" | sed --posix -e 'a\bar'
foo
bar

[e-kwsm]

a\bar is not specified in POSIX; it is a GNU extension. BSD rejects it:

sed: 1: "a\bar
": extra characters after \ at the end of a command