posix: don't process events when read returns < 0

[catfact]

i'm using libmonome to read directly from a serial device on linux.

much to my surprise, at a certain point in the project i started getting segfaults and spurious callbacks on device disconnect, which i never saw before.

currently my test device is a latest-edition 128.

after digging a bit, i found that next_event() is returning -1 (ultimately from here), but the event loop just checks for logical zero, so it continues with a bogus event structure, which may or may not blow up spectacularly.

here's a minimal patch that works for me, but maybe there's a better solution

[wrl]

Nope, looks good to me. monome_platform_read() can definitely return a negative, so this was just sloppy coding on my part.

Thanks.