Added explicit casts to malloc usages in order to fix compilation on C++

[GlitchedPolygons]

When compiling this Argon2 lib in a C++ project, the compilation fails due to illegal conversions from void* (what malloc(size_t) returns) to uint8_t* - this is perfectly valid in C (and don't get me wrong, I love C just as much as everyone else) but it's a non-ignorable error in C++.

Since you put

#if defined(__cplusplus)
extern "C" {
#endif

inside argon2.h I thought I'd open this pull request, because it makes no sense to hint C++ friendliness with that extern "C" if compilation then fails in C++ due to implicit pointer type conversions.

My idea here would be to

• Either accept this pull request and merge full C++ compatibility into Argon2 lib with minimal changes
• Or reject this pull request but then also remove the #ifdef _cplusplus block at the top of the header file to avoid confusion for C++ devs interested in using this library

Kindest regards, and have a happy and successful new year! :D

[Hello71]

although I am not argon2 author, extern "C" is fully consistent and in fact mandatory. argon2 is a C library that can be used from C++. it is not a C++ library. almost all C++ implementations except the notoriously deficient Microsoft implementation can be configured to compile certain files in C compliant mode, and even the Microsoft implementation now has official C compliant mode. extern "C" means that the referenced functions are written in C or more precisely use the C ABI, but can be seamlessly invoked from C++.

[josephlr]

@Hello71 is correct. The #if defined(__cplusplus) stuff means that the headers for this library are C++ compatible. The fact that the implementation files for this library end in .c indicate that the library itself should be built with a C compiler not a C++ compiler.

I think this PR can be closed unless we think that adding the explicit casts is good C style. C++ compatibility for the .c files should be a non-goal in my opinion.