[libc++] random_device fails if open returns zero

random_device::random_device(const string&) wrongly assumes that open
can only validly return a file descriptor greater than zero.

This results in random_device believing that it didn't successfully open
the device causing it to throw in it's constructor, this ends up leaking
a file descriptor.

The fix is simple, don't error on file descriptors which are zero.

llvm-svn: 210060