The format of the roareadiff output was recently changed, in
[99a8ae9d: "linux-ramdump-parser-v2: Update roareadiff for arm64"].
That change introduced the possibility of printing non-printable
characters in string representations of the memory dumps.  Fix this by
outputting a `.' for non-printable characters.

Change-Id: I1efd0775fac89618f43986523c6a9207afed7e5e