fix: avoid racy use of atomic counter

Don't know what I was smoking in 08f4ce1 ("fix: use C11 atomic
functions for concurrent counter updates", 2024-08-17), but I had
concurrent threads doing this:

| Thread A      | Thread B      |
| ------------- | ------------- |
| Read counter  | Read counter  |
| Write counter | Write counter |

Author: wincent

lua/wincent/commandt/lib/matcher.c

@@ -190,12 +190,10 @@ result_t *commandt_matcher_run(matcher_t *matcher, const char *needle) {
         if (i == worker_count - 1) {
             // For the last worker, we'll just use the main thread.
             heap_t *heap = get_matches(&worker_args[i]);
+            unsigned offset = atomic_fetch_add(&matches_count, heap->count);
             memcpy(
-                matches + atomic_load(&matches_count),
-                heap->entries,
-                heap->count * sizeof(haystack_t *)
+                matches + offset, heap->entries, heap->count * sizeof(haystack_t *)
             );
-            atomic_fetch_add(&matches_count, heap->count);
             heap_free(heap);
         } else {
             int err = pthread_create(
@@ -213,12 +211,10 @@ result_t *commandt_matcher_run(matcher_t *matcher, const char *needle) {
         if (err != 0) {
             die("phtread_join() failed", err);
         }
+        unsigned offset = atomic_fetch_add(&matches_count, heap->count);
         memcpy(
-            matches + atomic_load(&matches_count),
-            heap->entries,
-            heap->count * sizeof(haystack_t *)
+            matches + offset, heap->entries, heap->count * sizeof(haystack_t *)
         );
-        atomic_fetch_add(&matches_count, heap->count);
         heap_free(heap);
     }