RFR (XS): 6358357: Division by zero in Threads tab when shrinking jconsole window

[Erik Gahlin]

Please review a small change:

Description:
Switch to the Threads tab and try shrinking the window. When the window 
width becomes smaller than some particular size, an exception is printed 
to the terminal on every window update.

Bug:
http://monaco.us.oracle.com/detail.jsf?cr=6358357

Webrev:
http://cr.openjdk.java.net/~egahlin/6358357/

Testing:
Verified that no exceptions occur.

A sponsor is needed.

Thanks
Erik