Tiny fix for folly::Random::secureOneIn(0) case

Summary: Same code as in oneIn() explicit check for arg 0, 1.

Reviewed By: yfeldblum

Differential Revision: D25838957

fbshipit-source-id: 36ead588e435f116d0870ba45a181979c591c3f2

folly/Random.h

@@ -159,6 +159,9 @@ class Random {
    * Returns true 1/n of the time. If n == 0, always returns false
    */
   static bool secureOneIn(uint32_t n) {
+    if (n < 2) {
+      return n;
+    }
     SecureRNG<uint32_t> srng;
     return rand32(0, n, srng) == 0;
   }

folly/test/RandomTest.cpp

@@ -145,7 +145,9 @@ TEST(Random, sanity) {
 TEST(Random, oneIn) {
   for (auto i = 0; i < 10; ++i) {
     EXPECT_FALSE(folly::Random::oneIn(0));
+    EXPECT_FALSE(folly::Random::secureOneIn(0));
     EXPECT_TRUE(folly::Random::oneIn(1));
+    EXPECT_TRUE(folly::Random::secureOneIn(1));
   }
 
   // When using higher sampling rates, we'll just ensure that we see both