Don't purge junk filled chunks when shrinking huge allocations

When junk filling is enabled, shrinking an allocation fills the bytes
that were previously allocated but now aren't. Purging the chunk before
doing that is just a waste of time.

This resolves #260.

test/integration/chunk.c

@@ -1,5 +1,9 @@
 #include "test/jemalloc_test.h"
 
+#ifdef JEMALLOC_FILL
+const char *malloc_conf = "junk:false";
+#endif
+
 static chunk_hooks_t orig_hooks;
 static chunk_hooks_t old_hooks;