Fix unsafe unwrap in rewind

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "charred"
-version = "0.2.1"
+version = "0.2.2"
 authors = ["trivernis <trivernis@protonmail.com>"]
 edition = "2018"
 license-file = "LICENSE"

src/tapemachine.rs

@@ -91,8 +91,10 @@ impl CharTapeMachine {
     /// Rewinds to a given index
     #[inline]
     pub fn rewind(&mut self, index: usize) {
-        self.index = index;
-        self.current_char = *self.text.get(index).unwrap();
+        if self.text.len() > index {
+            self.index = index;
+            self.current_char = *self.text.get(index).unwrap();
+        }
     }
 
     /// Rewinds to a given index and returns an error