Fix a bug "use of logical '&&' with constant" in bench .c

This had been introduced with commit cc4650b1-
Looks like init() had been called for all formats, not just for dynamic formats.

clang warned about this:
bench.c:437:28: warning: use of logical '&&' with constant operand [-Wconstant-logical-operand]
                if (format->params.flags && FMT_DYNAMIC)
                                         ^  ~~~~~~~~~~~
bench.c:437:28: note: use '&' for a bitwise operation
                if (format->params.flags && FMT_DYNAMIC)
                                         ^~
                                         &
bench.c:437:28: note: remove constant to silence this warning
                if (format->params.flags && FMT_DYNAMIC)
                                        ~^~~~~~~~~~~~~~
1 warning generated.

Also, adjusted the comment, while at it (added a FIXME).