Fix issue #53 -- "local variable 'prop'"

[DrYak]

More extended fix to the crashes that did appear when window didn't contain enough sequences, leading to no output of dpm_sampler.

• Fix: missing null check that crashed dpm_sampler
• Fix: no samples that crashed shotgun.py
• at line 279: use 'not found' instead of zero, because line 618 checks for that too
• at line 332: also check for empty windows when merging the corrected.