libfetch: Use memcpy in place of an odd strncpy.

The length passed to strncpy is the length of the source string, not
the destination buffer. This triggers a non-fatal warning in GCC 12.
Hoewver, the code is also odd. It is really just a memcpy of the
string without its nul terminator. For that use case, memcpy is
clearer.

Reviewed by: imp, emaste
Differential Revision: https://reviews.freebsd.org/D36824