Fix wrongly ordered cleanup.

Warning was:

```
In function ‘mp_clear’,
    inlined from ‘mp_gcd.part.0’ at mp_gcd.c:89:4:
mp_clear.c:10:9: warning: ‘v.dp’ may be used uninitialized [-Wmaybe-uninitialized]
   10 |    if (a->dp != NULL) {
      |         ^
```

Fixes: aab1b3d ("split label & statement")

Signed-off-by: Steffen Jaeckel <[email protected]>

Author: sjaeckel

mp_gcd.c

@@ -84,9 +84,9 @@ mp_err mp_gcd(const mp_int *a, const mp_int *b, mp_int *c)
    c->sign = MP_ZPOS;
    err = MP_OKAY;
 LBL_V:
-   mp_clear(&u);
-LBL_U:
    mp_clear(&v);
+LBL_U:
+   mp_clear(&u);
    return err;
 }
 #endif