Fix a 100%-CPU bug where an SSL connection would sometimes never stop trying to write

Fix a 100%-CPU bug where an SSL connection would sometimes never stop trying to write

If an SSL connection becamse disabled or suspended before became open,
it could (under the right circumstances) wind up without ever getting
its write callback disabled.

The most correct fix is probably more subtle, and involves checking
all caseswhen a write callback is enabled or disabled.  This fix is
more blunt, and explicitly checks whether the callback should have
been disabled at the end of the callback to prevent infinite looping.

Diagnosed with help from Sebastian Hahn