BUG/MINOR: pattern: Do not pass len = 0 to calloc()

The behavior of calloc() when being passed `0` as `nelem` is implementation
defined. It may return a NULL pointer.

Avoid this issue by checking before allocating. While doing so adjust the local
integer variables that are used to refer to memory offsets to `size_t`.

This issue was introced in commit f91ac19299. This
patch should be backported together with that commit.

src/pattern.c

@@ -2660,10 +2660,10 @@ static int cmp_pat_ref(const void *_a, const void *_b)
  */
 int pattern_finalize_config(void)
 {
-	int len = 0;
-	int unassigned_pos = 0;
+	size_t len = 0;
+	size_t unassigned_pos = 0;
 	int next_unique_id = 0;
-	int i, j;
+	size_t i, j;
 	struct pat_ref *ref, **arr;
 	struct list pr = LIST_HEAD_INIT(pr);
 
@@ -2676,6 +2676,10 @@ int pattern_finalize_config(void)
 			unassigned_pos++;
 	}
 
+	if (len == 0) {
+		return 0;
+	}
+
 	arr = calloc(len, sizeof(*arr));
 	if (arr == NULL) {
 		ha_alert("Out of memory error.\n");