Added a fallback solution if having troubles with dev/urandom. Discovered a...

Added a fallback solution if having troubles with dev/urandom. Discovered a case where RHEL4 returned one more byte than requested...

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@851 44740490-163a-0410-bde0-09ae8108e29a