test_audio_cvt crash on Windows bundled release

[waych]

test_audio_cvt is crashing on Windows bundled dynamic build with heap corruption:

process didn't exit successfully: D:\a\rust-sdl2\rust-sdl2\target\release\deps\sdl2-3261d2c91077dec6.exe (exit code: 0xc0000374, STATUS_HEAP_CORRUPTION)

[waych]

Discussed in #1094 and #1092

[waych]

Commenting out the call the sys::SDL_ConvertAudio is enough to make the test pass again, suggesting that something is off with this bit of unsafe code.

In fact, the whole thing works correctly for me if I simply add assert_eq!(raw.buf, src.as_mut_ptr()); directly after the call, suggesting that the way the Vec's buffer is used here is not sound.

[Cobrand]

The data conversion may go through several passes; any given pass may possibly temporarily increase the size of the data. For example, SDL might expand 16-bit data to 32 bits before resampling to a lower frequency, shrinking the data size after having grown it briefly. Since the supplied buffer will be both the source and destination, converting as necessary in-place, the application must allocate a buffer that will fully contain the data during its largest conversion pass.

From the docs of SDL_ConvertAudio, that might be the problem. Since we send a rust Vec buffer, if it tries to re-allocate because the sample is too small, that means it expects to deallocate and reallocate our rust buffer using SDL_free and SDL_malloc, causing the error then.

This error doesn't happen on Linux, so this logic must be different depending on the OS we use (which makes sense because audio systems are deeply tied to the OS), which would be why it hasn't been caught until now.

So the solution here would be to change how the buffer sent to SDL_ConvertAudio is allocated, we have to send a SDL_malloc allocated one and never a rust-allocated one.