Fixed bug in handling exceptions raised during connection establishment.

doc/src/release_notes.rst

@@ -18,6 +18,7 @@ Thin Mode Changes
 #)  Internal changes to improve handling of multiple address and description
     lists in full connect descriptors.
 #)  Fixed bug in handling database response in certain unusual circumstances.
+#)  Fixed bug in handling exceptions raised during connection establishment.
 
 Thick Mode Changes
 ++++++++++++++++++

src/oracledb/impl/thin/protocol.pyx

@@ -340,7 +340,7 @@ cdef class Protocol:
         Forces the connection closed. This is used when an unrecoverable error
         has taken place.
         """
-        if self._socket is not None:
+        if self._socket is not None and self._read_buf._socket is not None:
             sock = self._socket
             if DEBUG_PACKETS:
                 now = datetime.datetime.now()