#20013: don't raise socket error when selected mailbox deleted.

I'm checking this in without a test because not much of this code
is tested and I don't have time to work up the necessary extensions
to the existing test framework.

The patch itself was tested by the person who reported the bug.
diff --git a/Lib/imaplib.py b/Lib/imaplib.py
index 3f6656a..ad5f4e9 100644
--- a/Lib/imaplib.py
+++ b/Lib/imaplib.py
@@ -1063,6 +1063,11 @@
                 del self.tagged_commands[tag]
                 return result
 
+            # If we've seen a BYE at this point, the socket will be
+            # closed, so report the BYE now.
+
+            self._check_bye()
+
             # Some have reported "unexpected response" exceptions.
             # Note that ignoring them here causes loops.
             # Instead, send me details of the unexpected response and