honor Espressif fp_int used length in s_fp_add operands

wolfcrypt/src/tfm.c

@@ -138,9 +138,31 @@ int s_fp_add(fp_int *a, fp_int *b, fp_int *c)
  oldused = MIN(c->used, FP_SIZE); /* help static analysis w/ largest size */
  c->used = y;
 
- t = 0;
+ t = 0; /* Our running total starts at zero. */
  for (x = 0; x < y; x++) {
- t += ((fp_word)a->dp[x]) + ((fp_word)b->dp[x]);
+ if ( (x < a->used) && (x < b->used) ) {
+ /* x is less than both [a].used and [b].used, so we add both */
+ t += ((fp_word)a->dp[x]) + ((fp_word)b->dp[x]);
+ }
+ else {
+ /* Here we honor the actual [a].used and [b].used values
+ * and NOT assume that values beyond [used] are zero. */
+ if ((x >= a->used) && (x < b->used)) {
+ /* x more than [a].used, [b] ok, so just add [b] */
+ t += /* ((fp_word)(0)) + */ ((fp_word)b->dp[x]);
+ }
+ else {
+ if ((x < a->used) && (x >= b->used)) {
+ /* x more than [b].used, [a] ok, so just add [a] */
+ t += ((fp_word)a->dp[x]) /* + (fp_word)(0) */;
+ }
+ else {
+ /* we should never get here, as a.used cannot be greater
+ * than b.used, while b.used is greater than a.used! */
+ /* t += 0 + 0 */
+ } /* nothing added */
+ } /* else only a added */
+ } /* else only a or only b added */
  c->dp[x] = (fp_digit)t;
  t >>= DIGIT_BIT;
  }