ssh2_exec: skip error outputs for EAGAIN

Since the example uses non-blocking mode, it will just flood the output
with this "nonsense" error.

example/ssh2_exec.c

@@ -264,6 +264,8 @@ int main(int argc, char *argv[])
                 fprintf(stderr, "\n");
             }
             else {
+                if( rc != LIBSSH2_ERROR_EAGAIN )
+                    /* no need to output this for the EAGAIN case */
                     fprintf(stderr, "libssh2_channel_read returned %d\n", rc);
             }
         }