Resolve "memory corruption bugs in libsane"

Merged Olaf Meeuwissen requested to merge 279-issue02-null-pointer-deref-epsonds_net_read into master

This adds fixes for security issues pointed out in #279 (closed).

In 1.0.30 we opted to disable the epsonds backend's network autodiscovery to mitigate security issues because it does not support network scanners. The changes here were implemented before realizing that 🤦 but they are useful in case someone wants to have a go add making it work.

/cc @kevinbackhouse