Address GCC 9.1 -O3 compilation warnings

Address GCC 9.1 -O3 compilation warnings

Compiling with -O3 triggers the following warnings with GCC 9.1:

    task.c: In function ‘isc_taskmgr_create’:
    task.c:1386:43: warning: ‘%04u’ directive output may be truncated writing between 4 and 10 bytes into a region of size 6 [-Wformat-truncation=]
     1386 |   snprintf(name, sizeof(name), "isc-worker%04u", i);
          |                                           ^~~~
    task.c:1386:32: note: directive argument in the range [0, 4294967294]
     1386 |   snprintf(name, sizeof(name), "isc-worker%04u", i);
          |                                ^~~~~~~~~~~~~~~~
    task.c:1386:3: note: ‘snprintf’ output between 15 and 21 bytes into a destination of size 16
     1386 |   snprintf(name, sizeof(name), "isc-worker%04u", i);
          |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    private_test.c: In function ‘private_nsec3_totext_test’:
    private_test.c:113:9: warning: array subscript 4 is outside array bounds of ‘uint32_t[1]’ {aka ‘unsigned int[1]’} [-Warray-bounds]
      113 |  while (*sp == '\0' && slen > 0) {
          |         ^~~
    private_test.c:106:11: note: while referencing ‘salt’
      106 |  uint32_t salt;
          |           ^~~~

Prevent these warnings from being triggered by increasing the size of
the relevant array (task.c) and reordering conditions (private_test.c).

(cherry picked from commit ce796ac1f4bf6c64ad0e8be937933309e6942c83)