rust/pv: fix Invalid write of size 1

Fix a valgrind finding. Fix an invalid read/write of one byte after the
actual struct to clear. Not fixing this may result in a illegal write or
memory corruption of the program. Fortunately, for the actual only user,
pvsecret this is not the case.

Fixes: c6f621d0 ("rust: Add library for pv tools")
Signed-off-by: Steffen Eiden <seiden@linux.ibm.com>

rust/pv/src/secret.rs

@@ -34,8 +34,8 @@ impl Zeroize for Vec<u8> {
             // * Vec allocated at least capacity elements continuously
             // * dst points always to a valid location
             unsafe {
+                std::ptr::write_volatile(dst, 0);
                 dst = dst.add(1);
-                std::ptr::write_volatile(dst, 0)
             }
         }
         std::sync::atomic::compiler_fence(std::sync::atomic::Ordering::SeqCst);