[ISDN]: Static overruns in drivers/isdn/i4l/isdn_ppp.c

Coverity found some static overruns in isdn_ppp.c (bug id #519) At several
places slot is compared <0 and > ISDN_MAX_CHANNELS and then used to index
ippp_table[ISDN_MAX_CHANNELS] A value of slot = ISDN_MAX_CHANNELS would run
over the end of the array.

Signed-off-by: Eric Sesterhenn <snakebyte@gmx.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>