[lldb] Support "absolute memory address" images in crashlog.py

The binary image list contains the following entry when a frame is not
found in any know binary image:

  {
    "size" : 0,
    "source" : "A",
    "base" : 0,
    "uuid" : "00000000-0000-0000-0000-000000000000"
  }

Note that this object is missing the name and path keys. This patch
makes the JSON parser resilient against their absence.