Anker Charger 45W for Macbook Pro 15 inch 2017
Yes, it will be safe.

When using lower capacity chargers the risk is primarily to the charger, not to your MacBook Pro. However, it's mainly only quite low capacity chargers (e.g. those used for some smartphones, etc) that are going to be at risk.

In the case of the Anker PowerPort+ 5 Ports USB-C, it's actually a 60W USB wall charger and is specifically designed to charge devices such as your MacBook. According to their product description page, it is designed to charge the MacBook at a full 29W speed-almost twice as fast as a standard USB-C charge (note that by standard they're not referring to Apple's MBP chargers).

What you will find in comparison to your Apple charger is that it will charge the battery slower. How much slower will depend on a number of variables, such as whether your computer is sleeping/shutdown or whether it's actively being used. And, if it is being used, it will depend on what you're doing. If you're doing CPU intensive tasks (e.g. video editing, playing graphics intensive games, etc) then it will charge slower again.

However, in a nutshell, your MBP battery will be totally fine.

Finally, you may be interested in reading up on what Apple says about it's batteries. And you may find coconutBattery useful. It provides a whole heap of information that you'll find useful, and from memory includes both Discharging with and Charging with measurements in watts!

NOTE: I am not affiliated with coconutBattery in any way.