Fix crash when passing filenames on command-line

[adriaandegroot]

Consider running diamond file.txt. If previously there
was an untitled tab open and nothing else, we arrive
here with 2 tabs, cnt==2. The first for-loop finds
an untitled tab at index k==0 and decrements cnt,
then the for-loop increments k and the for-loop terminates
(because 1 < 1 is false). We have cnt==1 but an empty
list m_openedFiles. This crashes with an out-of-bounds access
in the second for-loop, because cnt doesn't match the length
of the list anymore.

As a fix:

• do not modify cnt in the first for-loop, always check
  all of the current tabs,
• re-calculate the cnt based on the files that are actually
  opened, before the second loop.