[PATCH] x86_64: fix a use-after-free bug in -e option

There was a use-after-free in find_vmemmap_x86_64(), which is used
for the -e opion, and it can cause a segmentation fault when malloc()
is used later:

  #0  0x00007f70fa6724b9 malloc (libc.so.6 + 0x8c4b9)
  #1  0x0000556b3d1fbe6b prepare_bitmap1_buffer (makedumpfile + 0x38e6b)
  #2  0x0000556b3d208912 create_dump_bitmap (makedumpfile + 0x45912)
  #3  0x0000556b3d20e8c7 create_dumpfile (makedumpfile + 0x4b8c7)
  #4  0x0000556b3d1ce0ec main (makedumpfile + 0xb0ec)
  #5  0x00007f70fa60db75 __libc_start_main (libc.so.6 + 0x27b75)
  #6  0x0000556b3d1ce54e _start (makedumpfile + 0xb54e)

Signed-off-by: Kazuhito Hagio <k-hagio-ab@nec.com>