arm64: LLVMLinux: Check for NULL in futext_atomic_cmpxchg_inatomic()

futex_init passes in uaddr = 0. The LLVM optimizer will see that *uaddr is 0, set the register
to xzr and compilation will fail.
Signed-off-by: Mark Charlebois <charlebm@gmail.com>

arch/arm64/include/asm/futex.h

@@ -108,7 +108,7 @@ futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr,
 	int ret = 0;
 	u32 val, tmp;
 
-	if (!access_ok(VERIFY_WRITE, uaddr, sizeof(u32)))
+	if (uaddr == 0 || !access_ok(VERIFY_WRITE, uaddr, sizeof(u32)))
 		return -EFAULT;
 
 	asm volatile("// futex_atomic_cmpxchg_inatomic\n"