Heapmath mp_add_d: fix for when a and c same pointer

When parameters a and c to mp_add_d are the same pointer, c->sign was
being set to zero/positive and then a->sign was being checked.
Set the c->sign at end as it will always be zero/positive through the
code and the sign of the result isn't otherwise used.

wolfcrypt/src/integer.c

@@ -4425,9 +4425,6 @@ int mp_add_d (mp_int* a, mp_digit b, mp_int* c) /* //NOLINT(misc-no-recursion) *
   /* old number of used digits in c */
   oldused = c->used;
 
-  /* sign always positive */
-  c->sign = MP_ZPOS;
-
   /* source alias */
   tmpa    = a->dp;
 
@@ -4478,6 +4475,9 @@ int mp_add_d (mp_int* a, mp_digit b, mp_int* c) /* //NOLINT(misc-no-recursion) *
      ix       = 1;
   }
 
+  /* sign always positive */
+  c->sign = MP_ZPOS;
+
   /* now zero to oldused */
   while (ix++ < oldused) {
      *tmpc++ = 0;