Fix null dereference after failed channel lookup

When looking up the channel object for the current channel ID, if the
lookup failed, we still checked if the channel had an EOF with a null
pointer. That function, does check for NULL and error, but it is better
to error out sooner.

Affected function: ReceiveScpMessage.

Author: ejohnstown

src/wolfscp.c

@@ -1423,9 +1423,10 @@ int ReceiveScpMessage(WOLFSSH* ssh)
         if (err == 0) {
             WOLFSSH_CHANNEL* channel;
             channel = wolfSSH_ChannelFind(ssh, lastChannel, WS_CHANNEL_ID_SELF);
-            if (channel == NULL)
+            if (channel == NULL) {
                 ret = WS_INVALID_CHANID;
-            if (wolfSSH_ChannelGetEof(channel)) {
+            }
+            else if (wolfSSH_ChannelGetEof(channel)) {
                 return WS_EOF;
             }
         }