Fix post-return compilation warning

src/preserves.nim

@@ -1171,10 +1171,11 @@ proc fromPreserve*[T, E](v: var T; pr: Preserve[E]): bool {.gcsafe.} =
     result = fromPreserveHook(v, pr)
       # a previous branch determined that the hook does not compile but
       # calling it here explicitly produces a reasonable compiler error
-  if not result:
-    trace T, " !- ", pr
-  else:
-    trace T, " <- ", pr
+  when defined(tracePreserves):
+    if not result:
+      trace T, " !- ", pr
+    else:
+      trace T, " <- ", pr
 
 proc preserveTo*(pr: Preserve; T: typedesc): Option[T] =
   ## Reverse of `toPreserve`.