Codegen can break because toCommentSafeString is not actually safe

toCommentSafeString method replaces "\u" with "\ \u" to avoid codegen breaking.
But if the even number of "\" is put before "u", like "\ \u", in the string literal in the query, codegen can break.

Following code causes compilation error.

val df = Seq(...).toDF
df.select("'\\\\\\\\u002A/'").show

The reason of the compilation error is because "\\\\\\\\u002A/" is translated into "*/" (the end of comment).

Due to this unsafety, arbitrary code can be injected like as follows.

val df = Seq(...).toDF
// Inject "System.exit(1)"
df.select("'\\\\\\\\u002A/{System.exit(1);}/*'").show