RISC-V: Avoid dereferening NULL regs in die()

I don't think we can actually die() without a regs pointer, but the
compiler was warning about a NULL check after a dereference.  It seems
prudent to just avoid the possibly-NULL dereference, given that when
die()ing the system is already toast so who knows how we got there.
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20220920200037.6727-1-palmer@rivosinc.comSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/kernel/traps.c

@@ -33,6 +33,7 @@ void die(struct pt_regs *regs, const char *str)
 {
 	static int die_counter;
 	int ret;
+	long cause;
 
 	oops_enter();
 
@@ -42,11 +43,13 @@ void die(struct pt_regs *regs, const char *str)
 
 	pr_emerg("%s [#%d]\n", str, ++die_counter);
 	print_modules();
-	show_regs(regs);
+	if (regs)
+		show_regs(regs);
 
-	ret = notify_die(DIE_OOPS, str, regs, 0, regs->cause, SIGSEGV);
+	cause = regs ? regs->cause : -1;
+	ret = notify_die(DIE_OOPS, str, regs, 0, cause, SIGSEGV);
 
-	if (regs && kexec_should_crash(current))
+	if (kexec_should_crash(current))
 		crash_kexec(regs);
 
 	bust_spinlocks(0);