x86: kexec/i386: fix sparse warnings: Using plain integer as NULL pointer

Fix these sparse warnings:

  arch/x86/kernel/machine_kexec_32.c:124:22: warning: Using plain integer as NULL pointer
  arch/x86/kernel/traps.c:950:24: warning: Using plain integer as NULL pointer

Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Cc: trivial@kernel.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>