LT-14634, Illegal chars in pic file should not crash sort

Various things used to crash calling GetFullPathFromRelativeLFPath
if somehow a file name containing invalid characters got into our
picture file names. It would be ideal to prevent this happening,
but at least this fix prevents crashes resulting, and gives the
user a fairly clear indication that something is wrong.

Change-Id: I37fb511e78b0a2e596ec8cd299e895f8d8abc1fa