use volatile correctly

src/libdisk/proto.c

diff --git a/src/libdisk/proto.c b/src/libdisk/proto.c
index 8c94ddc..d1b136d 100644
--- a/src/libdisk/proto.c
+++ b/src/libdisk/proto.c
@@ -78,7 +78,7 @@
 {
 	Mkaux mx, *m;
 	File file;
-	int rv;
+	volatile int rv;
 
 	m = &mx;
 	memset(&mx, 0, sizeof mx);
@@ -101,7 +101,7 @@
 	file.new = "";
 	file.old = nil;
 
-	*(&rv) = 0;
+	rv = 0;
 	if(setjmp(m->jmp) == 0)
 		domkfs(m, &file, -1);
 	else