Fix panic with invalid unicode query (#645)

Without this fix the panic looks like the following:

```rust
---- parser::tests::lexer::string_errors stdout ----
thread 'parser::tests::lexer::string_errors' panicked at 'byte index 4 is not a char boundary; it is inside 'ɠ' (bytes 3..5) of `"\uɠ^A`', src/libcore/str/mod.rs:2219:5
```

This was found via fuzzing with `cargo-fuzz`.

juniper/src/parser/lexer.rs

@@ -296,7 +296,11 @@ impl<'a> Lexer<'a> {
             len += 1;
         }
 
-        let escape = &self.source[start_idx..=end_idx];
+        // Make sure we are on a valid char boundary.
+        let escape = &self
+            .source
+            .get(start_idx..=end_idx)
+            .ok_or_else(|| Spanning::zero_width(&self.position, LexerError::UnterminatedString))?;
 
         if len != 4 {
             return Err(Spanning::zero_width(

juniper/src/parser/tests/lexer.rs

@@ -322,6 +322,15 @@ fn string_errors() {
             LexerError::UnterminatedString
         )
     );
+
+    // Found by fuzzing.
+    assert_eq!(
+        tokenize_error(r#""\uɠ^A"#),
+        Spanning::zero_width(
+            &SourcePosition::new(5, 0, 5),
+            LexerError::UnterminatedString
+        )
+    );
 }
 
 #[test]