webclient: dispose the response body by default

Instead of a NULL dereference.

This fixes a crash on CONNECT failure when trying to proxy https.

netutils/webclient/webclient.c

@@ -1976,7 +1976,7 @@ int webclient_perform(FAR struct webclient_context *ctx)
                               goto errout_with_errno;
                             }
                         }
-                      else
+                      else if (ctx->callback)
                         {
                           ctx->callback(&ws->buffer, ws->offset,
                                         ws->offset + received,