Fix gcc-10.1 C++20 warnings

[masterleinad]

Accompanying #3716. This fixes

warning: ‘char* strncpy(char*, const char*, size_t)’ specified bound 120 equals destination size [-Wstringop-truncation]

which was the only warning with -std=c++2a using gcc-10.1.
Note that we set the last element to \0 anyway in the next line.

[nliber]

Yeah, I saw that it was correct when it was checked in before, even if gcc can't see that. :-). Anyway, this change is fine.

[masterleinad]

Ready from my side. Needs more reviews.

[Rombur]

This looks good. Do you mind also unifying the way we set the last character. Right now, we use (char) 0, \0, and 0.