Tests: test_index: Make it clear that my_alloc() has no integer overflows

Tests: test_index: Make it clear that my_alloc() has no integer overflows

liblzma guarantees that the product of the allocation size arguments
will fit in size_t.

Putting the pre-increment in the if-statement was clearly wrong
although in practice it didn't matter here as the function is
called only a couple of times.