Bug: FreePrintBuf doesn't null values

I'm not sure if this is what is intended, but FreePrintBuf() should have
these two lines added:
// clear the buffer values so they can be used again
linebuf = NULL ;
lbufsize = 0 ;

this will then allow tidy to be called again and again within other code if
it isn't run directly from a command prompt and the code is integrated into
a larger application.

...Stefan