wolfsshd: save shellBuffer on WS_WANT_WRITE

ChannelIdSend / extended_data_send returning WS_WANT_WRITE set
wantWrite but didn't save cnt_r, so the next read overwrote the
held bytes. Store cnt_r in windowFull at all three read sites
(stderr, stdout, pty childFd); flag windowFullExt for stderr so
the retry routes through extended_data_send.

Author: ejohnstown

apps/wolfsshd/wolfsshd.c

@@ -1691,6 +1691,8 @@ static int SHELL_Subsystem(WOLFSSHD_CONNECTION* conn, WOLFSSH* ssh,
                             continue;
                         }
                         else if (cnt_w == WS_WANT_WRITE) {
+                            windowFull = cnt_r;
+                            windowFullExt = 1;
                             wantWrite = 1;
                             continue;
                         }
@@ -1729,6 +1731,7 @@ static int SHELL_Subsystem(WOLFSSHD_CONNECTION* conn, WOLFSSH* ssh,
                             continue;
                         }
                         else if (cnt_w == WS_WANT_WRITE) {
+                            windowFull = cnt_r;
                             wantWrite = 1;
                             continue;
                         }
@@ -1765,6 +1768,7 @@ static int SHELL_Subsystem(WOLFSSHD_CONNECTION* conn, WOLFSSH* ssh,
                             continue;
                         }
                         else if (cnt_w == WS_WANT_WRITE) {
+                            windowFull = cnt_r;
                             wantWrite = 1;
                             continue;
                         }