rewrite_comment: fix block fallback when failing to rewrite an itemized block
rewrite_comment: fix block fallback when failing to rewrite an itemized block

Close #3224

aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,rust-lang/rustfmt,graydon/rust,graydon/rust,SingingTree/rustfmt,aidancully/rust,graydon/rust,aidancully/rust,rust-lang/rustfmt,graydon/rust,aidancully/rust,graydon/rust,SingingTree/rustfmt