Fix junk filling of cached large objects.

Use the size argument to tcache_dalloc_large() to control the number of
bytes set to 0x5a when junk filling is enabled, rather than accessing a
non-existent arena bin.  This bug was capable of corrupting an
arbitrarily large memory region, depending on what followed the arena
data structure in memory (typically zeroed memory, another arena_t, or a
red-black tree node for a huge object).