isdn/capi: prevent NULL pointer dereference on invalid CAPI command
isdn/capi: prevent NULL pointer dereference on invalid CAPI command

An invalid CAPI 2.0 command/subcommand combination may retrieve a
NULL pointer from the cpars[] array which will later be dereferenced
by the parser routines.
Fix by adding NULL pointer checks in strategic places.

Signed-off-by: Tilman Schmidt <dc8b785d67b4628425f4b275427621124af5d812@imap.cc>
Signed-off-by: David S. Miller <fe08d3c717adf2ae63592e4c9aec6e3e404d8e3e@davemloft.net>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs