raop: Fix infinite loop in raop-sink with UDP mode

There are potentially unresolved issues around the EAGAIN logic, but
this should prevent an infinite loop and log flood in the mean time.

Fixes: #695
2 jobs for master in 4 minutes and 36 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #511748
build-autotools

00:04:36

passed #511749
build-meson

00:01:11