rewrite_string: do not overflow the next line when the line_start is not a whitespace
rewrite_string: do not overflow the next line when the line_start is not a whitespace

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