Handle out of memory by exiting instead of segfaulting

[sirainen]

Dovecot limits process VSZ by default, and we noticed that libexttextcat crashes when it can't allocate memory. Segfaulting doesn't make it very clear why the failure happened, so would be nice to get some kind of an error message printed. Here's the simplest patch I thought of to fix it.

[kloczek]

Any chance merge this PR and release new version? 🤔