Fix attach of a previously-detached injection point.

It's normal for the name in a free slot to match the new name.  The
max_inuse mechanism kept simple cases from reaching the problem.  The
problem could appear when index 0 was the previously-detached entry and
index 1 is in use.  Back-patch to v17, where this code first appeared.

src/backend/utils/misc/injection_point.c

@@ -315,8 +315,7 @@ InjectionPointAttach(const char *name,
 			if (free_idx == -1)
 				free_idx = idx;
 		}
-
-		if (strcmp(entry->name, name) == 0)
+		else if (strcmp(entry->name, name) == 0)
 			elog(ERROR, "injection point \"%s\" already defined", name);
 	}
 	if (free_idx == -1)