radeonfb_parse_monitor_layout() will produce an array overrun if passed with a
substring of length higher than 4 (ie, "XXXXX,YYYYYY").

Coverity Bug 494
Signed-off-by: Antonino Daplas <adaplas@pol.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>