fix: missing initialization of sample parameters

[wbruna]

• the value of the parameter eta was undefined

• replace memset calls on parameter struct initializations with simpler initialization lists

• add a missing free on an error path in the context initialization

[Green-Sky]

Memsetting some types is undefined or implementation defined behavior and should be avoided. Since this is kinda c++, wouldn't something like *sample_params = {}; work better?

[wbruna]

It is less of an issue for these parameter structs, though, since they are supposed to be C-compatible. But I fully agree type-safe initializations would be better; I just followed the way the other init functions are doing it.

Edit: replaced the memsets, and rebased onto master.

[leejet]

Thank you for your contribution.