Calling wifi.scan() twice on ESP8266 board causes failure

[nkolban]

After calling wifi.scan(), calling it a second time causes an assertion crash.

[nkolban]

The problem was easily recreated. It was tracked down to the logic which saves the callback function to be invoked when the scan for access points completes. The scan should invoke the callback function (which it does) and then it should unlock the JsVar which holds the callback function (which it does) ... however it then failed to NULL the JsVar. Later, when the wifi.scan() was called a second time, it saw that the previous save of the callback function (which is now unlocked) was not NULL and tried to UnLock it again ... which failed ... because it is already unlocked.

The correction was to NULL the JsVar after unlocking it after invoking the callback.

[nkolban]

Changes made, committed and pushed.