suggested method to preserve intent of drop to fix error

However, I didn't write this library (and I don't have a test
suite for it), so I'm not quite sure if this preserves the
invariants on the fd_mapping() structure. Needs @xobs' review.

services/pddb/src/libstd/mod.rs

@@ -517,7 +517,6 @@ pub(crate) fn close_key(
             return Err(crate::PddbRetcode::UnexpectedEof);
         }
     };
-    core::mem::drop(file);
 
     // If this SID is unused, close the connection.
     if let Some(cid) = conn {

services/pddb/src/main.rs

@@ -1096,6 +1096,7 @@ fn wrapped_main() -> ! {
                         if let Err(e) = result {
                             xous::return_scalar(msg.sender, e as usize)
                         } else {
+                            fd_mapping.remove(&msg.sender.pid());
                             xous::return_scalar2(msg.sender, 0, 0)
                         }.ok();
                     }