bsonrw.valueReader does not verify length of string before slice for CodeWithScope

Within the ReadCodeWithScope method of bsonrw.valueReader we don't check the length of strBytes before attempting to remove the null byte. This can cause a panic if the BSON is invalid and strLength is 0.

To fix this we need to check the length of strBytes and if it's 0 we need to return an error because the BSON is invalid.

Thanks to @dgryski for raising this.