Make format_ucs2() not use a variable-length array.

Make format_ucs2() not use a variable-length array.

Coverity complains (during the build, as a "recoverable" error):

"dp.h", line 134: warning #1234: a variable-length array is not allowed inside
          of a statement expression
                uint16_t _ucs2buf[(len)];                               \
                         ^

So don't do that.

Signed-off-by: Peter Jones <pjones@redhat.com>