Avoid UB in enc_ma.cc.
Avoid UB in enc_ma.cc.

Creating a geometric distribution with fraction 0 causes divisions by 0
and invalid float-to-int conversions. Fixed #409.

libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl