maint: avoid signed overflow warning with -O3

maint: avoid signed overflow warning with -O3

Prompted by the implicit -O3 added by american-fuzzy-lop,
seen with GCC 4.9.2 on x86_64.

  src/pr.c: In function 'print_files.part.5':
  src/pr.c:1781:6: error: assuming signed overflow does not occur
  when simplifying conditional to constant [-Werror=strict-overflow]
     if (cols_ready_to_print () == 0)

This happens because cols_ready_to_print() is inlined
thus reducing the comparison to the N variable in print_page().
Now this can't overflow due to the protection when parsing the
specified column, but use an unsigned type to avoid the
apparent signed overflow.

* src/pr.c (cols_ready_to_print): Increment an unsigned type to
avoid the subsequent signed overflow warning.