[text] Ensure clutter_text_get_selection copes with start/end positions of -1

[text] Ensure clutter_text_get_selection copes with start/end positions of -1

I was seeing clutter_text_get_selection trying to malloc up to 4Gb due to
unexpected negative arithmetic for the start/end offsets which resulted
in a crash.

This just tests for positions of -1 before deciding if the start/end
positions need to be swapped.  The conversion from position to byte offset
already works with -1.