lib/securerandom.rb: test one byte to determine urandom or openssl

`SecureRandom#gen_random` determines whether urandom is available or not
by trying `Random.urandom(n)`.  But, when n = 0, `Random.urandom(0)`
always succeeds even if urandom is not available, which leads to a wrong
decision.

When failed, `Random.urandom` returns nil instead of returning a shorter
string than required.  So the check for `ret.length != n` is not needed.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59840 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/securerandom.rb

@@ -52,7 +52,7 @@ module SecureRandom
     end
 
     def gen_random(n)
-      ret = Random.urandom(n)
+      ret = Random.urandom(1)
       if ret.nil?
         begin
           require 'openssl'
@@ -67,10 +67,6 @@ module SecureRandom
           end
           return gen_random(n)
         end
-      elsif ret.length != n
-        raise NotImplementedError, \
-              "Unexpected partial read from random device: " \
-              "only #{ret.length} for #{n} bytes"
       else
         @rng_chooser.synchronize do
           class << self