Leave interactive mode on end of input

The interactive loop only ever broke on the literal string "quit".
std::getline sets failbit at end of input and leaves the string empty,
which the loop read as an ordinary blank command, so a piped script
without a trailing quit -- or Ctrl+Z at a console -- spun forever
printing prompts, and pawnio_close at the bottom of the function was
never reached.

Break when the stream fails, so end of input falls through to the
existing close and the VM is torn down the same way quit tears it down.

Measured by piping a single "ioctl_test 0" with no quit into both
builds and killing anything still alive after two seconds:

  before  still running, killed, 14004679 bytes of prompts on stdout
  after   exited 0 after 57 bytes

The documented sequence with an explicit quit still prints two
"received 0 cells:" lines and exits 0.