perror: don't overwrite rax if rax=0 (mov rax,r9 before .quit)

src/perror.asm

@@ -210,6 +210,6 @@ perror:
 	mov	rdi, FD_stderr
 	mov	rsi, r10
 	syscall
-	.quit:
 	mov	rax, r9
+	.quit:
 	ret