extmod/modlwip: Fix error return for TCP recv when not connected.

This commit fixes the cases when a TCP socket is in STATE_NEW,
STATE_LISTENING or STATE_CONNECTING and recv() is called on it.  It now
raises ENOTCONN instead of a random error code due to it previously
indexing beyond the start of error_lookup_table[].

Signed-off-by: Damien George <damien@micropython.org>

tests/extmod/usocket_tcp_basic.py

@@ -0,0 +1,17 @@
+# Test basic, stand-alone TCP socket functionality
+
+try:
+    import usocket as socket, uerrno as errno
+except ImportError:
+    try:
+        import socket, errno
+    except ImportError:
+        print("SKIP")
+        raise SystemExit
+
+# recv() on a fresh socket should raise ENOTCONN
+s = socket.socket()
+try:
+    s.recv(1)
+except OSError as er:
+    print("ENOTCONN:", er.args[0] == errno.ENOTCONN)