Prevent issues with stacks exactly 11 deep

I was seeing `panic serving 10.40.243.118:44663: runtime error: index
out of range` `/tmp/build_c0c3d66e-c25b-42a1-9bf2-a77b92b2f44c/.vendor/src/github.com/github/go-opstocat/haystack.go:36+0x65a` when attempting to report an exception via grohl + go-opstocat.

[haystack.go:36](https://github.com/github/go-opstocat/blob/7faea155fd76d1c43f9dd30622664674586cbbc1/haystack.go#L36) attempts to grab the first frame in the backtrace, but apparently the slice was empty!

I found this bit of code which, as it was previously written, returns an empty slice if the stack is exactly 11 frames deep.

I think the intention was to always grab the top 10 stack frames; if so, I think it can be rewritten like this. Reminder that go's slice range indices are exclusive: `[0:10]` means grab elements 0 through 9.

/cc: @technoweenie
I think it had to do with the first 10 lines being the same in every trace, or something.  But yeah maybe preserving all of them is the best thing to do.