TypeError on empty stacktrace

[iamed2]

               _
   _       _ _(_)_     |  A fresh approach to technical computing
  (_)     | (_) (_)    |  Documentation: http://docs.julialang.org
   _ _   _| |_  __ _   |  Type "?help" for help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 0.6.0-dev.1618 (2016-12-19 18:51 UTC)
 _/ |\__'_|_|_|\__'_|  |  Commit dce82fcbc* (0 days old master)
|__/                   |  x86_64-apple-darwin16.3.0

julia> catch_stacktrace()
ERROR: TypeError: stacktrace: in typeassert, expected Array{StackFrame,1}, got Array{Any,1}
 in stacktrace(::Array{Ptr{Void},1}, ::Bool) at ./stacktraces.jl:147
 in catch_stacktrace() at ./stacktraces.jl:166

Also happens on 0.5.

One-line PR fix incoming!