First, ellipsize() and ellipsize_mem() should not read past the input
buffer. Those functions take an explicit length for the input data, so they
should not assume that the buffer is terminated by a nul.
Second, ellipsization was off in various cases where wide on multi-byte
characters were used.
We had some basic test for ellipsize(), but apparently it wasn't enough to
catch more serious cases.
Should fix https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=8686.
