Fix TCP race condition, remove fixed delay in CC (#5167)

ClientContext::_wait_for_sent() could dereference a TCP's _pcb after
the connection was dropped by the OS, resulting in a crash.

Move the connection dropped check to catch this case, and replace
a fixed millisecond delay() with a yield and timeout value to minimize
wasted time when transmission completes.

Author: earlephilhower

libraries/ESP8266WiFi/src/include/ClientContext.h

@@ -308,39 +308,40 @@ class ClientContext
         if (!_pcb)
             return true;
 
-        int loop = -1;
         int prevsndbuf = -1;
-        max_wait_ms++;
 
         // wait for peer's acks to flush lwIP's output buffer
-
+        uint32_t last_sent = millis();
         while (1) {
+            if (millis() - last_sent > (uint32_t) max_wait_ms) {
+#ifdef DEBUGV
+                // wait until sent: timeout
+                DEBUGV(":wustmo\n");
+#endif
+                // All data was not flushed, timeout hit
+                return false;
+            }
 
             // force lwIP to send what can be sent
             tcp_output(_pcb);
 
             int sndbuf = tcp_sndbuf(_pcb);
             if (sndbuf != prevsndbuf) {
                 // send buffer has changed (or first iteration)
-                // we received an ack: restart the loop counter
                 prevsndbuf = sndbuf;
-                loop = max_wait_ms;
+                // We just sent a bit, move timeout forward
+                last_sent = millis();
             }
 
-            if (state() != ESTABLISHED || sndbuf == TCP_SND_BUF || --loop <= 0)
-                break;
-
-            delay(1);
-        }
+            yield();
 
-        #ifdef DEBUGV
-        if (loop <= 0) {
-            // wait until sent: timeout
-            DEBUGV(":wustmo\n");
+            if ((state() != ESTABLISHED) || (sndbuf == TCP_SND_BUF)) {
+                break;
+            }
         }
-        #endif
 
-        return max_wait_ms > 0;
+        // All data flushed
+        return true;
     }
 
     uint8_t state() const