maint: mbsalign: fix an edge case where we truncate too much

maint: mbsalign: fix an edge case where we truncate too much

* gl/lib/mbsalign.c (mbsalign): Ensure the temporary destination buffer
is big enough, as it may need to be bigger than the source buffer
in the presence of single byte non printable chars.
* gl/tests/test-mbsalign.c (main): Add a test to trigger the issue.