Modified the validation for ascii tag type

src/error.rs

@@ -1,6 +1,7 @@
 use std::error::Error;
 use std::fmt;
 use std::io;
+use std::str;
 use std::string;
 
 use crate::decoder::ifd::Value;
@@ -246,6 +247,12 @@ impl From<io::Error> for TiffError {
     }
 }
 
+impl From<str::Utf8Error> for TiffError {
+    fn from(_err: str::Utf8Error) -> TiffError {
+        TiffError::FormatError(TiffFormatError::InvalidTag)
+    }
+}
+
 impl From<string::FromUtf8Error> for TiffError {
     fn from(_err: string::FromUtf8Error) -> TiffError {
         TiffError::FormatError(TiffFormatError::InvalidTag)