Fix c2c2c_identity test

Reusing a plan for different arrays than it was created with requires that they
have the same size and alignment as was used for creating the plan. While the
size of `a` and `b` is identical, there is no such guarantee for their
alignment. Therefore one may not simply swap them.

Fix this by using separate plans for the `in: a, out: b` and `in: b, out: a`
invocations.

Author: LingMan

fftw/tests/c2c.rs

@@ -8,13 +8,18 @@ fn c2c2c_identity() {
     let n = 32;
     let mut a = vec![c64::zero(); n];
     let mut b = vec![c64::zero(); n];
-    let mut plan: C2CPlan64 =
+
+    let mut plan_ab: C2CPlan64 =
         C2CPlan::new(&[n], &mut a, &mut b, Sign::Forward, Flag::MEASURE).unwrap();
+    let mut plan_ba: C2CPlan64 =
+        C2CPlan::new(&[n], &mut b, &mut a, Sign::Forward, Flag::MEASURE).unwrap();
+
     for i in 0..n {
         a[i] = c64::new(1.0, 0.0);
     }
-    plan.c2c(&mut a, &mut b).unwrap();
-    plan.c2c(&mut b, &mut a).unwrap();
+
+    plan_ab.c2c(&mut a, &mut b).unwrap();
+    plan_ba.c2c(&mut b, &mut a).unwrap();
     for v in a.iter() {
         let ans = c64::new(n as f64, 0.0);
         let dif = (v - ans).norm();