m68k/mm: Check for mm != NULL in do_page_fault() debug code

When DEBUG is enabled, do_page_fault() may dereference a NULL pointer,
causing recursive bus errors.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
diff --git a/arch/m68k/mm/fault.c b/arch/m68k/mm/fault.c
index eb1d61f..c7725f1 100644
--- a/arch/m68k/mm/fault.c
+++ b/arch/m68k/mm/fault.c
@@ -77,8 +77,7 @@
 
 #ifdef DEBUG
 	printk ("do page fault:\nregs->sr=%#x, regs->pc=%#lx, address=%#lx, %ld, %p\n",
-		regs->sr, regs->pc, address, error_code,
-		current->mm->pgd);
+		regs->sr, regs->pc, address, error_code, mm ? mm->pgd : NULL);
 #endif
 
 	/*