random-util: handle if getrandom() returns 0

This should normally not happen, but given that the man page suggests
something about this in the context of interruption, let's handle this
and propagate an I/O error.

src/basic/random-util.c

@@ -93,6 +93,9 @@ int genuine_random_bytes(void *p, size_t n, bool high_quality_required) {
                         }
 
                         already_done = r;
+                } else if (r == 0) {
+                        have_syscall = true;
+                        return -EIO;
                 } else if (errno == ENOSYS)
                           /* We lack the syscall, continue with reading from /dev/urandom. */
                           have_syscall = false;