MunitParameterEnum with NULL value at the beginning leads to segfault#88

`MunitParameterEnum` is not correctly processed by `munit_suite_main`. If value parameter is `NULL` and located at the beginning of the list it will lead to segfault. ``` static MunitParameterEnum test_params[] = { { (char*) "foo", NULL }, // NULL instead of foo_params { (char*) "bar", bar_params }, { (char*) "baz", NULL }, { NULL, NULL }, }; ``` --- ``` gdb example run ... Running test suite with seed 0x9e8678ec... /example/compare [Detaching after fork from child process 20542] [ OK ] [ 0.00000232 / 0.00000146 CPU ] /example/rand [Detaching after fork from child process 20543] [ OK ] [ 0.00000495 / 0.00000472 CPU ] /example/parameters Program received signal SIGSEGV, Segmentation fault. 0x0000555555558705 in munit_test_runner_run_test_wild () (gdb) bt #0 0x0000555555558705 in munit_test_runner_run_test_wild () #1 0x0000555555558b78 in munit_test_runner_run_test () #2 0x0000555555558d45 in munit_test_runner_run_suite () #3 0x0000555555558e27 in munit_test_runner_run () #4 0x000055555555a05d in munit_suite_main_custom () #5 0x000055555555a255 in munit_suite_main () #6 0x000055555555adef in main () ```