coverity CID 352930, fix for out of bounds write

Author: JacobBarthelmeh

wolfcrypt/src/tfm.c

@@ -1104,9 +1104,9 @@ void fp_mod_2d(fp_int *a, int b, fp_int *c)
 
    bmax = ((unsigned int)b + DIGIT_BIT - 1) / DIGIT_BIT;
 
-   /* If a is negative and bmax is larger than FP_SIZE, then the
+   /* If a is negative and bmax is greater than or equal to FP_SIZE, then the
     * result can't fit within c. Just return. */
-   if (c->sign == FP_NEG && bmax > FP_SIZE) {
+   if (c->sign == FP_NEG && bmax >= FP_SIZE) {
       return;
    }