test/func/malloc fixes

- `check_result()` `abort()` on `null` and non-`nullptr` result.  Otherwise it
  just prints and doesn't end the test

- Don't call `realloc(, 0)`; this has never been consistent and the current C2x
  draft (see §7.22.3.5 and N2464) finally just declares it to be undefined
  behavior.  POSIX (2017) tolerates our current behavior of freeing the
  passed-in pointer and returning a new object.

src/test/func/malloc/malloc.cc

@@ -20,9 +20,8 @@ void check_result(size_t size, size_t align, void* p, int err, bool null)
     if (p != nullptr)
     {
       printf("Expected null, and got non-null return!\n");
-      failed = true;
+      abort();
     }
-    our_free(p);
     return;
   }
 
@@ -183,7 +182,6 @@ int main(int argc, char** argv)
   {
     const size_t size = sizeclass_to_size(sc);
     test_realloc(our_malloc(size), size, SUCCESS, false);
-    test_realloc(our_malloc(size), 0, SUCCESS, true);
     test_realloc(nullptr, size, SUCCESS, false);
     test_realloc(our_malloc(size), ((size_t)-1) / 2, ENOMEM, true);
     for (sizeclass_t sc2 = 0; sc2 < NUM_SIZECLASSES; sc2++)
@@ -198,7 +196,6 @@ int main(int argc, char** argv)
   {
     const size_t size = bits::one_at_bit(sc);
     test_realloc(our_malloc(size), size, SUCCESS, false);
-    test_realloc(our_malloc(size), 0, SUCCESS, true);
     test_realloc(nullptr, size, SUCCESS, false);
     test_realloc(our_malloc(size), ((size_t)-1) / 2, ENOMEM, true);
     for (sizeclass_t sc2 = 0; sc2 < (MAX_SIZECLASS_BITS + 4); sc2++)