justify range to prevent negative tab span and memory alloc, rhbz#1057741

I could not reproduce the crash of that bug (probably having more memory
available), but the backtrace had nTabSpan = -2 implicitly casted to
size_t leading to allocation of a huge amount of memory with
vector::reserve(), which ScRange::Justify() exactly prevents.

Change-Id: Idb79e1be62649922ba793cab01e00011479fade9