FFI function cleanup_openal_context() has a memory issue without pre-checking

[cicilzx]

Hi, I'm developing a fuzzer to test the safe abstraction, and I fonnd a segmentation fault when calling function ears/src/internal.rs/cleanup_openal_context().

The test case is as follows:

#[allow(warnings, unused)]
#[test]
fn test_cleanup_openal_context_s7Huz() {
    let _ = cleanup_openal_context();
}

Calling the cleanup_openal_context() function directly without any pre-checks results in a memory error, which is reported as follows:

Caused by:
  process didn't exit successfully: `/home/cici/ears/target/debug/deps/ears-3aa3cfab0315f845 'internal::test_cleanup_openal_context_s7Huz' --exact --show-output` (signal: 11, SIGSEGV: invalid memory reference)