Revert back to only ffi-level checking of open context

The Rust context is always open, the return value of this function was
simply the wrong way around.

deltachat-ffi/src/lib.rs

@@ -276,9 +276,11 @@ pub unsafe extern "C" fn dc_is_open(context: *mut dc_context_t) -> libc::c_int {
         return 0;
     }
     let ffi_context = &*context;
-    ffi_context
-        .with_inner(|ctx| ctx.sql.is_open() as libc::c_int)
-        .unwrap_or(0)
+    let inner_guard = ffi_context.inner.read().unwrap();
+    match *inner_guard {
+        Some(_) => 1,
+        None => 0,
+    }
 }
 
 #[no_mangle]