Hooks: Add a hook exhaustion test.

When we run out of space in which to store hooks, we should return EAGAIN from
the mallctl, but not otherwise misbehave.

include/jemalloc/internal/hook.h

@@ -46,6 +46,8 @@
  *   order.
  */
 
+#define HOOK_MAX 4
+
 enum hook_alloc_e {
 	hook_alloc_malloc,
 	hook_alloc_posix_memalign,