Commit c8ea87e4 introduced a temporary conversion buffer for
substrings extracted during regexp splits. Unfortunately the code that
sized it was failing to ignore the effects of ignored degenerate
regexp matches, so for regexp_split_* calls it could under-size the
buffer in such cases.

Fix, and add some regression test cases (though those will only catch
the bug if run in a multibyte encoding).

Backpatch to 9.3 as the faulty code was.

Thanks to the PostGIS project, Regina Obe and Paul Ramsey for the
report (via IRC) and assistance in analysis. Patch by me.