[plt-scheme] mzscheme enters infinite loop when unable to write to stdout

mzscheme enters an infinite loop printing "error writing to stream
port" at the REPL if it is unable to write to stdout.  (the odd
position of the ">" prompt in the output is due to redirecting stdout
to a pipe).

$ mzscheme | head -n 5
Welcome to MzScheme v4.1.5 [3m], Copyright (c) 2004-2009 PLT Scheme Inc.
1
> 1
2
> 2
3
> 3
4
> 4
5
> 5
error writing to stream port (Broken pipe; errno=32)

error writing to stream port (Broken pipe; errno=32)

error writing to stream port (Broken pipe; errno=32)

error writing to stream port (Broken pipe; errno=32)

error writing to stream port (Broken pipe; errno=32)

error writing to stream port (Broken pipe; errno=32)