lib: tls: _pusher return -1 when ssl is nil
lib: tls: _pusher return -1 when ssl is nil

follow the logic of nodejs here. I was getting backtraces from
tests/test-tls-client-reject.lua because a socket that failed to
authozire was calling :write()

rjeli/luvit,bsn069/luvit,zhaozg/luvit,AndrewTsao/luvit,GabrielNicolasAvellaneda/luvit-upstream,luvit/luvit,rjeli/luvit,rjeli/luvit,GabrielNicolasAvellaneda/luvit-upstream,sousoux/luvit,sousoux/luvit,kaustavha/luvit,bsn069/luvit,sousoux/luvit,AndrewTsao/luvit,boundary/luvit,DBarney/luvit,kaustavha/luvit,boundary/luvit,GabrielNicolasAvellaneda/luvit-upstream,DBarney/luvit,boundary/luvit,kaustavha/luvit,luvit/luvit,sousoux/luvit,connectFree/lev,boundary/luvit,zhaozg/luvit,DBarney/luvit,GabrielNicolasAvellaneda/luvit-upstream,AndrewTsao/luvit,sousoux/luvit,boundary/luvit,connectFree/lev,rjeli/luvit,sousoux/luvit,DBarney/luvit,boundary/luvit,bsn069/luvit