Don't copy input to new buffer if it is the FLUSH sentinel

I ran into an error which this patch fixes:

    TypeError: Got stream error: Object #<Object> has no method 'copy'
        at Object.convert (/home/anders/projects/foo/node_modules/iconv/lib/iconv.js:101:11)
        at Iconv.write (/home/anders/projects/foo/node_modules/iconv/lib/iconv.js:61:17)
        at Iconv.end (/home/anders/projects/foo/node_modules/iconv/lib/iconv.js:77:10)
        ...

Unfortunately I have not been able to write a simple script that reproduces the error, but the symptoms I see are as follows.

I have a sequence of streams piped into one another, where the Iconv stream is the last.  After data
has been sent iconv.convert is called with the first argument being an empty object.