Fix/improve gtk_text_buffer_get_iter_at_*() functions
Submitted by Sébastien Wilmet
As bug #735337 shows, it would be nice to make gtk_text_buffer_get_iter_at_line_index() a bit nicer in case of an invalid index, instead of crashing.
A warning could be printed, or the function could have a boolean return value indicating whether the exact position has been found.
In any case the function should return the iter at the end of the line. Or the end iter if the line number is greater than the total number of lines.
This applies to the other related functions in GtkTextBuffer, like gtk_text_buffer_get_iter_at_line_offset().
Also, gtk_text_buffer_get_iter_at_line() has a related bug when the line_number is too big, it should return the end iter in this case, but if the last line of the buffer is not empty, it returns the iter at offset 0 of this last line, instead of the end of the last line.