network-stack: Fix racy TCP data loss upon peer closure

Where the closure resulted in us letting go of the PCB, and the recv()
logic would return early because the PCB was gone. At that point there
might still be data left in the RX buffer, but the application wouldn't
see it.

src/fruity/network-stack.vala

@@ -601,9 +601,6 @@ namespace Frida.Fruity {
 				ssize_t n = 0;
 
 				netstack.perform_on_lwip_thread (() => {
-					if (pcb == null)
-						return OK;
-
 					lock (state) {
 						n = ssize_t.min (buffer.length, rx_buf.len);
 						if (n == 0)
@@ -612,6 +609,9 @@ namespace Frida.Fruity {
 						rx_buf.remove_range (0, (uint) n);
 					}
 
+					if (pcb == null)
+						return OK;
+
 					size_t remainder = n;
 					while (remainder != 0) {
 						uint16 chunk = (uint16) size_t.min (remainder, uint16.MAX);