So the phrase is saying that the person should be careful not to talk about Africa in certain circumstances or there is a possibility that someone (or some people) will beat them so severely that there will be no trace of Africa left in them.
The beating may be physical but obviously it's not actually possible to beat someone until they lose their nationality or racial origin.
I will point out that even though you are quoting from a book (and I believe a rather well-respected one), the use of the word "n*gger" is seen as extremely offensive in most circles these days. The fact that it is contained within a quote lessens this impact somewhat but be aware of this fact.
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