Optimize prof_occurrences by using pointer arithmetic and
g_utf8_next_char instead of calls to g_utf8_offset_to_pointer.
Improve list construction efficiency in prof_occurrences by
using g_slist_prepend and g_slist_reverse instead of appends.
Also fix a slight oversight that could lead to trouble:
When empty search strings (e.g. empty nicks) are passed
it caused matches at every position.
Update the test case to test for empty nicks.
ea7a59808a