Fix WiFiClientSecure::available() blocking on dropped connections

[johnm545]

When a secure socket connection is dropped somewhere between the access point and the server, write attempts eventually fill the WiFiClient's tx buffer. At this point, calls to WiFiClientSecure::available() block for 5 (default) seconds. See here for full description and example: #6434

This happens because WiFiClientSecure::available() calls _run_until(BR_SSL_RECVAPP, false), and this also triggers a blocking write attempt if there is still record data hanging around from previous write attempts (i.e. state & BR_SSL_SENDREC == true). The fix is to avoid writing to the WiFiClient more data than will fit in its tx buffer when the blocking flag == false, by first checking WiFiClient::availableForWrite(). This also works fine when availableForWrite() == 0 (WiFiClient::write just returns 0 instantly if the size argument is 0).

I have also removed the if (blocking) guard from the optimistic_yield call here. A yield is required to avoid WDT reset if available() is called in a busy loop (as is common). The WiFiClient::available() method includes a yield, so including one in the WiFiClientSecure method is consistent.

[earlephilhower]

LGTM, Thanks

[johnm545]

I've just pushed a fix for #6464 here too, as it's also a dropped connection issue and changes are in the same method

[d-a-v]

LGTM

[earlephilhower]

Thanks, LGTM now.