tc_exec: don't dereference NULL on calloc failure

tc_exec: don't dereference NULL on calloc failure

Reported as:
tc_exec.c: In function ‘do_exec’:
tc_exec.c:103:18: warning: dereference of NULL ‘eu’ [CWE-476] [-Wanalyzer-null-dereference]
  103 |         return eu->parse_eopt(eu, argc, argv);
      |                ~~^~~~~~~~~~~~
  ‘do_exec’: events 1-6
    |
    |   81 | int do_exec(int argc, char **argv)
    |      |     ^~~~~~~
    |      |     |
    |      |     (1) entry to ‘do_exec’
    |......
    |   86 |         if (argc < 1) {
    |      |            ~
    |      |            |
    |      |            (2) following ‘false’ branch (when ‘argc > 0’)...
    |......
    |   91 |         if (matches(*argv, "help") == 0) {
    |      |            ~~~~~~~~~~~~~~~~~~~~~~~
    |      |            ||
    |      |            |(3) ...to here
    |      |            (4) following ‘true’ branch...
    |......
    |   96 |         strncpy(kind, *argv, sizeof(kind) - 1);
    |      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    |      |         |
    |      |         (5) ...to here
    |   97 |
    |   98 |         eu = get_exec_kind(kind);
    |      |              ~~~~~~~~~~~~~~~~~~~
    |      |              |
    |      |              (6) calling ‘get_exec_kind’ from ‘do_exec’
    |
    +--> ‘get_exec_kind’: events 7-10
           |
           |   40 | static struct exec_util *get_exec_kind(const char *name)
           |      |                          ^~~~~~~~~~~~~
           |      |                          |
           |      |                          (7) entry to ‘get_exec_kind’
           |......
           |   63 |         if (eu == NULL)
           |      |            ~
           |      |            |
           |      |            (8) following ‘true’ branch (when ‘eu’ is NULL)...
           |   64 |                 goto noexist;
           |      |                 ~~~~
           |      |                 |
           |      |                 (9) ...to here
           |......
           |   72 |         if (eu) {
           |      |            ~
           |      |            |
           |      |            (10) following ‘false’ branch (when ‘eu’ is NULL)...
           |
         ‘get_exec_kind’: event 11
           |
           |cc1:
           | (11): ...to here
           |
    <------+
    |
  ‘do_exec’: events 12-13
    |
    |   98 |         eu = get_exec_kind(kind);
    |      |              ^~~~~~~~~~~~~~~~~~~
    |      |              |
    |      |              (12) return of NULL to ‘do_exec’ from ‘get_exec_kind’
    |......
    |  103 |         return eu->parse_eopt(eu, argc, argv);
    |      |                ~~~~~~~~~~~~~~
    |      |                  |
    |      |                  (13) dereference of NULL ‘eu’
    |

Fixes: 4bd624467bc6 ("tc: built-in eBPF exec proxy")
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>