Wrap next_code_point in unsafe block

Fixes #368
diff --git a/sgx_tstd/src/sys_common/wtf8.rs b/sgx_tstd/src/sys_common/wtf8.rs
index d37342a..dca7415 100644
--- a/sgx_tstd/src/sys_common/wtf8.rs
+++ b/sgx_tstd/src/sys_common/wtf8.rs
@@ -825,7 +825,9 @@
 
     #[inline]
     fn next(&mut self) -> Option<CodePoint> {
-        next_code_point(&mut self.bytes).map(|c| CodePoint { value: c })
+        unsafe {
+            next_code_point(&mut self.bytes).map(|c| CodePoint { value: c })
+        }
     }
 
     #[inline]