Fix double-free in pdumper

Revert the double-free bug that I introduced in
2019-03-11T15:20:54Z!eggert@cs.ucla.edu.
* src/pdumper.c (dump_mmap_reset): Do not free the private member;
that’s the release function’s job.
(dump_mm_heap_cb_release): Free cb if its refcount goes to zero.
(dump_mmap_contiguous_heap): Mention memory leak in comment.
1 job for master in 50 minutes and 48 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #1464
test-all

00:50:48