tfm fp_exptmod_nct: handle special cases better

Author: philljj

wolfcrypt/src/tfm.c

@@ -3171,16 +3171,21 @@ int fp_exptmod_nct(fp_int * G, fp_int * X, fp_int * P, fp_int * Y)
    int x = fp_count_bits (X);
 #endif
 
-   /* 0^X mod P = 0 mod P = 0.
-    * Set result to 0 and return early. */
-   if (fp_iszero(G)) {
+   /* handle modulus of zero and prevent overflows */
+   if (fp_iszero(P) || (P->used > (FP_SIZE/2))) {
+      return FP_VAL;
+   }
+   if (fp_isone(P)) {
       fp_set(Y, 0);
       return FP_OKAY;
    }
-
-   /* prevent overflows */
-   if (P->used > (FP_SIZE/2)) {
-      return FP_VAL;
+   if (fp_iszero(X)) {
+      fp_set(Y, 1);
+      return FP_OKAY;
+   }
+   if (fp_iszero(G)) {
+      fp_set(Y, 0);
+      return FP_OKAY;
    }
 
 #if defined(WOLFSSL_ESP32_CRYPT_RSA_PRI) && \