Fix clang-11 issues

Tested with clang 11.0.0-++20200715043845+0e377e253c1-1~exp1 on Debian sid

../src/network/test-networkd-conf.c:104:56: warning: adding 'int' to a string does not append to the string [-Wstring-plus-int]
        test_config_parse_duid_rawdata_one(BYTES_0_128 + 2, 0, &(DUID){0, 128, BYTES_1_128});
                                           ~~~~~~~~~~~~^~~
../src/network/test-networkd-conf.c:104:56: note: use array indexing to silence this warning
        test_config_parse_duid_rawdata_one(BYTES_0_128 + 2, 0, &(DUID){0, 128, BYTES_1_128});
                                                       ^
                                           &           [  ]
1 warning generated.

../src/test/test-clock.c:52:17: warning: ignoring return value of function declared with 'warn_unused_result' attribute [-Wunused-result]
                ftruncate(fileno(f), 0);
                ^~~~~~~~~ ~~~~~~~~~~~~
1 warning generated.

(gdb) run
Starting program: systemd/build/test-alloc-util
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
x1: 0x7fffffffd2d0
x2: 0x7fffffffdac0
y: 0x7fffffffd2cc
z: 0x7fffffffd2c0
cleanup2(0x7fffffffd2cc)
cleanup3(0x7fffffffd2c0)
cleanup1(0x7fffffffdac0)
cleanup1(0x7fffffffd2d0)
*** buffer overflow detected ***: terminated

Program received signal SIGABRT, Aborted.
__GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
50      ../sysdeps/unix/sysv/linux/raise.c: No such file or directory.
(gdb) bt full
        set = {__val = {0, 18446744073709551615, 0, 0, 0, 0, 140737348658240, 140737348659520, 0, 0, 0, 0, 0, 0, 0, 0}}
        pid = <optimized out>
        tid = <optimized out>
        ret = <optimized out>
        save_stage = 1
        act = {__sigaction_handler = {sa_handler = 0x22ff0, sa_sigaction = 0x22ff0}, sa_mask = {__val = {140737349888603, 4217127, 4217127, 4217127, 4217127, 4217383, 0, 0, 0, 0, 143329, 143344, 140737351162752, 8959, 18446744073709551328, 4289072}},
          sa_flags = -138460788, sa_restorer = 0xffffffffffffffff}
        sigs = {__val = {32, 0 <repeats 15 times>}}
        ap = {{gp_offset = 24, fp_offset = 0, overflow_arg_area = 0x7fffffffd280, reg_save_area = 0x7fffffffd210}}
        fd = <optimized out>
        list = <optimized out>
        nlist = <optimized out>
        cp = <optimized out>
No locals.
No locals.
No locals.
No locals.
No locals.
        l = 0
No locals.
        p1 = 0x405500 "\223Nd\n\351\301mA\214\262A\247\306b\276\317\327\353\346k\035\024\273{\276&!kλ\233\217\t\207\276\327\347\351\355\307R\276\063{\235w=\237E\357\277KL\245\374\245\066M\201+\333\064\272\332g>1<@"
        p2 = <optimized out>
        i = <optimized out>
No locals.