Remove BUG() in Windows process read callback.

This BUG() was added when the code was written to see if this callback
was ever executed after we marked the handle as EOF. It turns out, it
does, but we handle it gracefully. We can therefore remove the BUG().

Fixes tpo/core/tor#40596.

changes/ticket40596

@@ -0,0 +1,4 @@
+  o Minor bugfixes (pluggable transports, windows):
+    - Remove a warning `BUG()` that could occur when attempting to execute a
+      non-existing pluggable transport on Windows. Fixes bug 40596; bugfix on
+      0.4.0.1-alpha.

src/lib/process/process_win32.c

@@ -888,7 +888,7 @@ process_win32_read_from_handle(process_win32_handle_t *handle,
 
   /* Check if we have been asked to read from a handle that have already told
    * us that we have reached the end of the file. */
-  if (BUG(handle->reached_eof))
+  if (handle->reached_eof)
     return 0;
 
   /* This cast should be safe since our buffer can be at maximum up to