fix WiFiClient::availableForWrite() with no connexion (#3985)
fix WiFiClient::availableForWrite() with no connexion (#3985)

fix #3983
esp8266/Arduino,wemos/Arduino,sticilface/Arduino,wemos/Arduino,sticilface/Arduino,Lan-Hekary/Arduino,esp8266/Arduino,hallard/Arduino,Lan-Hekary/Arduino,sticilface/Arduino,Adam5Wu/Arduino,hallard/Arduino,sticilface/Arduino,lrmoreno007/Arduino,Lan-Hekary/Arduino,esp8266/Arduino,Adam5Wu/Arduino,hallard/Arduino,Adam5Wu/Arduino,hallard/Arduino,lrmoreno007/Arduino,wemos/Arduino,lrmoreno007/Arduino,sticilface/Arduino,wemos/Arduino,Lan-Hekary/Arduino,hallard/Arduino,Adam5Wu/Arduino,lrmoreno007/Arduino,esp8266/Arduino,Adam5Wu/Arduino,wemos/Arduino,esp8266/Arduino,Lan-Hekary/Arduino,lrmoreno007/Arduino