[PATCH] openpromfs: fix missing NUL

tchars is not '\0'-terminated so the strtoul may run into problems.  Fix that.
 Also make tchars as big as a long in hexadecimal form would take rather than
just 16.

Signed-off-by: Jan Engelhardt <jengelh@gmx.de>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>