Added check for wolfmath.c for digits == 0 and test for api.c

tests/api.c

@@ -33914,6 +33914,12 @@ static int test_mp_rand (void)
             ret = 0;
         }
     }
+    if (ret == 0) {
+        ret = mp_rand(&a, 0, &rng);
+        if (ret == BAD_FUNC_ARG) {
+            ret = 0;
+        }
+    }    
     if (ret == 0) {
         ret = mp_rand(&a, digits, &rng);
     }

wolfcrypt/src/wolfmath.c

@@ -146,7 +146,7 @@ int mp_rand(mp_int* a, int digits, WC_RNG* rng)
     if (rng == NULL) {
         ret = MISSING_RNG_E;
     }
-    else if (a == NULL) {
+    else if (a == NULL || digits == 0) {
         ret = BAD_FUNC_ARG;
     }