Catch a possible null-dereference

[einglis]

In testing, I had an HTTP server that would server a 200 OK header, then crash, killing the connection.

The result was an exception in HTTPUpdater, which turned out to be due to passing a null pointer to WiFiClient::stopAllExcept

In catching this, I've chosen to return HTTPC_ERROR_CONNECTION_LOST. This is an HTTPClient definition, rather than an HTTPUpdate one, which makes me a little uneasy. I've justified it to myself on the basis that:

1. There is no pre-existing HTTPUpdate value that seems appropriate, and
2. The ones that are defined are preceded by the comment: /// note we use HTTP client errors too [...]

[earlephilhower]

Thanks!