[MIPS] Check function pointers are non-zero before calling.

    
Several boards don't initialize the pointers, so let's play safe.
Signed-off-by: NRalf Baechle <ralf@linux-mips.org>

arch/mips/kernel/reset.c

@@ -23,16 +23,18 @@ void (*_machine_power_off)(void);
 
 void machine_restart(char *command)
 {
+	if (_machine_restart)
 		_machine_restart(command);
 }
 
 void machine_halt(void)
 {
+	if (_machine_halt)
 		_machine_halt();
 }
 
 void machine_power_off(void)
 {
+	if (_machine_power_off)
 		_machine_power_off();
 }
-