cgo panic on Mac OS X

[pphaneuf]

While fixing a small bug in our Travis configuration which ignored the result the of the Go tests (!!!) in #1151, it turns out that there's a strange error that seems to be only on on Mac OS X (but this might just be because of a different version of the Go compiler/runtime):

=== RUN   TestAddLeaf
--- FAIL: TestAddLeaf (0.00s)
panic: runtime error: cgo argument has Go pointer to Go pointer [recovered]
    panic: runtime error: cgo argument has Go pointer to Go pointer

goroutine 21 [running]:
panic(0x4178040, 0xc820062520)
    /usr/local/Cellar/go/1.6/libexec/src/runtime/panic.go:464 +0x3e6
testing.tRunner.func1(0xc82009c090)
    /usr/local/Cellar/go/1.6/libexec/src/testing/testing.go:467 +0x192
panic(0x4178040, 0xc820062520)
    /usr/local/Cellar/go/1.6/libexec/src/runtime/panic.go:426 +0x4e9
github.com/google/certificate-transparency/go/merkletree.(*CPPMerkleTree).CurrentRoot(0xc82007a0e0, 0x0, 0x0, 0x0, 0x0, 0x0)
    /Users/travis/build/google/install/go/src/github.com/google/certificate-transparency/go/merkletree/merkle_tree.go:55 +0x15e
github.com/google/certificate-transparency/go/merkletree.TestAddLeaf(0xc82009c090)
    /Users/travis/build/google/install/go/src/github.com/google/certificate-transparency/go/merkletree/merkle_tree_test.go:79 +0x481
testing.tRunner(0xc82009c090, 0x428c7b8)
    /usr/local/Cellar/go/1.6/libexec/src/testing/testing.go:473 +0x98
created by testing.RunTests
    /usr/local/Cellar/go/1.6/libexec/src/testing/testing.go:582 +0x892
FAIL    github.com/google/certificate-transparency/go/merkletree    0.036s

It failed in 4 of the 5 latest runs of the master branch, so it's a very frequent flaky test? This makes me scratch my head even more, because from what I understand of this error, it should be rather consistent?

This seems to be related to golang/go#12416.