Compat: 7.8::SSL: Some more robustness fixes in destroy(). Apply the "SSL: Some more robustness fixes in destroy()." fix from Pike 7.8 in the Pike 7.8 compat SSL module: set_nonblocking_keep_callbacks() calls update_internal_state(), which is likely to install callbacks and thus may trigger the tests in CHECK_CB_MODE() (cf [bug 6958] #6). Yet another potential fix for [bug 6958].