TTS engine crashes at 4th paragraph of nearly every article

# Issue details

### Duplicate?
no

### Actual behaviour
The TTS engine crashes after it reads the first 3 paragraph, I need to tap the stop button and then tap start button. The online tts engine I use is somehow fragile, and this problem is not found with another offline TTS engine. I believe the problem is due to some special character inserted by mistake, the character cause the crash.
The crashes dont occur at 100% rate, maybe more than 90%, and is not replated to the article or the articles from some particular sites.
The crashes dont occur only at the begining of the article, if I tap back to the article list, tap into the article and tap the play button, make the TTS read from where it stops, it may also crash.

### Expected behaviour
- Tell us what should happen

# Steps to reproduce the issue
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# Environment details
- **wallabag app version**: 2.5.1
- **wallabag app installation source**: F-Droid
- **Android OS version**: 10
- **Android ROM** (e.g. stock, LineageOS, SlimRom,…): Origin OS
- **Android hardware**: vivo X27
- **wallabag server version**: 2.6.6
- **Do you have Two-Factor-Authentication enabled?**: no

# Logs
## wallabag server
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## Web server
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# Your experience with wallabag Android app
Have you had any luck using wallabag Android app before? (Sometimes we get tired of reading bug reports all day and a lil' positive end note does wonders)