runtime: panic when accessing an empty struct value appended to an uninitialised slice

[0branch]

I ran into the following runtime error while attempting access an empty struct value that had been appended to an uninitialised slice: https://play.golang.org/p/J2ItiISh2f,

panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0xffffffff addr=0x0 pc=0x202e2]

The append (line 11) succeeds, as does printing the slice as a whole (line 12); accessing the appended struct via range (line 16) yields a panic. Issue encountered under 1.4.1; unable to replicate under gccgo.

Known workaround: comment out line 8 in favour of 9 and make an empty slice explicitly (yuck).

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The following also work as expected, though they alter intended behaviour:

• Modifying the struct to include a single field, allowing for default initialisation
• Taking a slice of pointers []*empty

[minux]

the problem is that append returns a slice with ptr=nil.
reproducible on tip.

[minux]

I've sent https://golang.org/cl/7400

[gopherbot]

CL https://golang.org/cl/14248 mentions this issue.