add unique RNG missing error

wolfcrypt/src/error.c

@@ -386,6 +386,9 @@ const char* wc_GetErrorString(int error)
     case ASN_COUNTRY_SIZE_E:
         return "Country code size error, either too small or large";
 
+    case MISSING_RNG_E:
+        return "RNG required but not provided";
+
     default:
         return "unknown error number";
 

wolfcrypt/src/rsa.c

@@ -812,7 +812,10 @@ static int mp_rand(mp_int* a, int digits, WC_RNG* rng)
     int ret;
     mp_digit d;
 
-    if (a == NULL || rng == NULL)
+    if (rng == NULL)
+        return MISSING_RNG_E;
+
+    if (a == NULL)
         return BAD_FUNC_ARG;
 
     mp_zero(a);

wolfssl/wolfcrypt/error-crypt.h

@@ -173,6 +173,7 @@ enum {
 
     WC_KEY_SIZE_E       = -234,  /* Key size error, either too small or large */
     ASN_COUNTRY_SIZE_E  = -235,  /* ASN Cert Gen, invalid country code size */
+    MISSING_RNG_E       = -236,  /* RNG required but not provided */
 
     MIN_CODE_E          = -300   /* errors -101 - -299 */