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

lib/libfetch/common.c

@@ -456,7 +456,7 @@ fetch_socks5_init(conn_t *conn, const char *host, int port, int verbose)
 		goto fail;
 	}
 	*ptr++ = strlen(host);
-	strncpy(ptr, host, strlen(host));
+	memcpy(ptr, host, strlen(host));
 	ptr = ptr + strlen(host);
 
 	port = htons(port);