Fix "Go pointer to Go pointer" panics

[evanj]

The Cgo runtime verifies that Go never passes pointers to other Go
pointers, which is required for correct garbage collection.
Unfortunately, its checks are not perfect, and there are occasional
false positives. Our code triggers these false positives if the
slice passed to compression functions is in the same memory
allocation as Go pointers. This happened when trying to use zstd with
another package's Writer type, which has an internal buffer.

The tests added in this PR all fail with the following panic. The
fix is to ensure the expression unsafe.Pointer(&src[0]) is inside the
Cgo call, and not before. This is documented in the following issue:

golang/go#14210 (comment)

The remaining uses of the "var srcPtr *byte" pattern are safe: they
all pass the address of a byte slice that is allocated internally by
this library, so I believe they cannot cause this error.

Fixes the following panic:

panic: runtime error: cgo argument has Go pointer to Go pointer

goroutine 30 [running]:
panic(...)
  /usr/local/go/src/runtime/panic.go:969 +0x1b9
github.com/DataDog/zstd.(*ctx).CompressLevel.func1(...)
  /home/circleci/project/zstd_ctx.go:75 +0xd9
github.com/DataDog/zstd.(*ctx).CompressLevel(...)
  /home/circleci/project/zstd_ctx.go:75 +0xce
github.com/DataDog/zstd.TestCtxCompressLevelNoGoPointers.func1(...)
  /home/circleci/project/zstd_ctx_test.go:71 +0x77
github.com/DataDog/zstd.testCompressNoGoPointers(...)
  /home/circleci/project/zstd_test.go:130 +0xad
github.com/DataDog/zstd.TestCtxCompressLevelNoGoPointers(...)
  /home/circleci/project/zstd_ctx_test.go:69 +0x37
testing.tRunner(...)
  /usr/local/go/src/testing/testing.go:1123 +0xef

[evanj]

I verified on CircleCI that the tests failed without the fix: https://app.circleci.com/pipelines/github/DataDog/zstd/90/workflows/d1eb9c40-deac-4358-81f4-ff148a65940b/jobs/333

Thanks to @niamster for telling me about this! This is an interesting one. Cgo is annoying.

[Viq111]

Thanks for the tests and the fix!

Would you mind making it not a draft PR anymore so we can merge it ?

I think @niamster tested it well so it should be good to go on this front as well

This should fix #98 once merged