kdb: Fix early debugging crash regression

The kdb_current legally be equal to NULL in the early boot of the x86
arch.  The problem pcan be observed by booting with the kernel arguments:

    earlyprintk=vga ekgdboc=kbd kgdbwait

The kdb shell will oops on entry and recursively fault because it
cannot get past the final stage of shell initialization.
Signed-off-by: NJason Wessel <jason.wessel@windriver.com>

kernel/debug/kdb/kdb_main.c

@@ -1127,7 +1127,7 @@ static int kdb_local(kdb_reason_t reason, int error, struct pt_regs *regs,
 		/* special case below */
 	} else {
 		kdb_printf("\nEntering kdb (current=0x%p, pid %d) ",
-			   kdb_current, kdb_current->pid);
+			   kdb_current, kdb_current ? kdb_current->pid : 0);
 #if defined(CONFIG_SMP)
 		kdb_printf("on processor %d ", raw_smp_processor_id());
 #endif