libfetch: Fix -Wunterminated-string-initialization

This defaults to an error in clang HEAD, use a char-by-char
initializer instead.

Reviewed by: emaste, jhb
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D52532

(cherry picked from commit 79f578531f21aa57307cbffd858a8a89e562aa5c)