shell_putc() prints chars twice when using buffered output

[joemcool]

shell_putc() calls both shell_writer() and obhandle->shell_bwriter() if both are not 0. This results in chars being printed twice, once by each writer. There is no way to avoid this condition when using buffered output, since shell_writer() is required by shell_init() and there's no way to undefined it.

Steps to reproduce:
--Initialize the shell as in the Shell_Basic example
--Add a buffered writer, something like:

void shell_buffered_writer(char* buffer, uint8_t count)  
{  
  // If we can write to a client  
  if(client==true && client.connected())  
  {  
    client.write(buffer, count);  
  }  
}

--Write enough output with shell_print() to trigger a buffered write
--Observe that the output is written twice

Proposed solution:
Rework the if statements in shell_putc() so that only one writer is called