Fix -Werror=array-bounds build failures with gcc 14.2 when MALLOC_0_RETURNS_NULL is defined

• _XlcDefaultMapModifiers: remove conversions between size_t & int
• _XimEncodingNegotiation: swap order of arguments to calloc
• _XimStrConversionCallback: use size_t to calculate size to malloc

The errors seem wrong to me, but since gcc 14.2 is already out in the wild, we need to work with it.