rd_kw_struct uses `char *data` which is UB.

[eivindjahren]

The rd_kw_struct uses char *data to store different types (int, float, double, bool), then casts it to typed pointers:

resdata/lib/resdata/rd_kw.cpp

Line 979 in 091b695

return (ctype *)rd_kw->data; \

Compilers assume pointers of different types don't alias, enabling aggressive optimizations that can break the code.

It should either

• Use std::aligned_storage and placement new
• Use a union
• Compile with -fno-strict-aliasing

We should probably also run tests with -fsanitize=undefined -fsanitize=address.