CID 1454593: passing negative value to memset

Coverity says:

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*** CID 1454593:  Memory - illegal accesses  (NO_EFFECT)
/src/test/test_util.c: 6200 in test_util_map_anon_nofork()
6194       int pipefd[2] = {-1, -1};
6195       unsigned inherit=0;
6196     
6197       tor_munmap_anonymous(ptr, sz);
6198       ptr = tor_mmap_anonymous(sz, ANONMAP_NOINHERIT, &inherit);
6199       tt_ptr_op(ptr, OP_NE, 0);
>>>     CID 1454593:  Memory - illegal accesses  (NO_EFFECT)
>>>     Argument "-48" in "memset" loses precision in "memset(ptr, -48, sz)".
6200       memset(ptr, TEST_VALUE, sz);
6201     
6202       tt_int_op(0, OP_EQ, pipe(pipefd));
6203       pid_t child = fork();
6204       if (child == 0) {
6205         /* We're in the child. */