Avoid unstable `inner_deref' feature

Author: Dmitry Bogatov

src/dc_chat.rs

@@ -1025,7 +1025,7 @@ unsafe fn set_draft_raw(context: &Context, chat_id: uint32_t, msg: *mut dc_msg_t
                     time(),
                     (*msg).type_0,
                     DC_STATE_OUT_DRAFT,
-                    (*msg).text.deref().unwrap_or(""),
+                    (*msg).text.as_ref().map(String::as_str).unwrap_or(""),
                     (*msg).param.to_string(),
                     1,
                 ],

src/lib.rs

@@ -1,4 +1,4 @@
-#![feature(c_variadic, ptr_wrapping_offset_from, ptr_cast, inner_deref)]
+#![feature(c_variadic, ptr_wrapping_offset_from, ptr_cast)]
 
 #[macro_use]
 extern crate failure_derive;