Heap math: mp_add/submod_ct make work when c == d

mp_addmod_ct and mp_submod_ct expected c and d to be different pointers.
Change code to support this use case.
Fix whitespace.

Author: SparkiDev

wolfcrypt/src/integer.c

@@ -3068,47 +3068,83 @@ int mp_submod(mp_int* a, mp_int* b, mp_int* c, mp_int* d)
 /* d = a + b (mod c) */
 int mp_addmod(mp_int* a, mp_int* b, mp_int* c, mp_int* d)
 {
-   int     res;
-   mp_int  t;
+  int     res;
+  mp_int  t;
 
-   if ((res = mp_init (&t)) != MP_OKAY) {
-     return res;
-   }
+  if ((res = mp_init (&t)) != MP_OKAY) {
+    return res;
+  }
 
-   res = mp_add (a, b, &t);
-   if (res == MP_OKAY) {
-       res = mp_mod (&t, c, d);
-   }
+  res = mp_add (a, b, &t);
+  if (res == MP_OKAY) {
+    res = mp_mod (&t, c, d);
+  }
 
-   mp_clear (&t);
+  mp_clear (&t);
 
-   return res;
+  return res;
 }
 
 /* d = a - b (mod c) - a < c and b < c and positive */
 int mp_submod_ct(mp_int* a, mp_int* b, mp_int* c, mp_int* d)
 {
-    int res;
+  int     res;
+  mp_int  t;
+  mp_int* r = d;
 
-    res = mp_sub(a, b, d);
-    if (res == MP_OKAY && mp_isneg(d)) {
-        res = mp_add(d, c, d);
+  if (c == d) {
+    r = &t;
+
+    if ((res = mp_init (r)) != MP_OKAY) {
+      return res;
     }
+  }
 
-    return res;
+  res = mp_sub (a, b, r);
+  if (res == MP_OKAY) {
+    if (mp_isneg (r)) {
+      res = mp_add (r, c, d);
+    } else if (c == d) {
+      res = mp_copy (r, d);
+    }
+  }
+
+  if (c == d) {
+    mp_clear (r);
+  }
+
+  return res;
 }
 
 /* d = a + b (mod c) - a < c and b < c and positive */
 int mp_addmod_ct(mp_int* a, mp_int* b, mp_int* c, mp_int* d)
 {
-    int res;
+  int     res;
+  mp_int  t;
+  mp_int* r = d;
+
+  if (c == d) {
+    r = &t;
 
-    res = mp_add(a, b, d);
-    if (res == MP_OKAY && mp_cmp(d, c) != MP_LT) {
-        res = mp_sub(d, c, d);
+    if ((res = mp_init (r)) != MP_OKAY) {
+      return res;
     }
+  }
 
-    return res;
+  res = mp_add (a, b, r);
+  if (res == MP_OKAY) {
+    if (mp_cmp (r, c) != MP_LT) {
+      res = mp_sub (r, c, d);
+    } else if (c == d) {
+      res = mp_copy (r, d);
+    }
+  }
+
+  if (c == d) {
+    mp_clear (r);
+  }
+
+  return res;
 }
 
 /* computes b = a*a */