stack overflow in format of `Rational{BigInt}`

[jlapeyre]

Formatting 0.2.0

julia> using Formatting

julia> format(big"1"//2, conversion="e")
ERROR: StackOverflowError:
 in ini_dec(::BigFloat, ::Int64) at ./printf.jl:953 (repeats 80000 times)

julia> versioninfo()
Julia Version 0.6.0-dev.435
Commit 8eed82e* (2016-08-30 13:50 UTC)
Platform Info:
  System: Linux (x86_64-linux-gnu)
  CPU: Intel(R) Core(TM) i7-4712HQ CPU @ 2.30GHz
  WORD_SIZE: 64
  BLAS: libopenblas (USE64BITINT DYNAMIC_ARCH NO_AFFINITY Haswell)
  LAPACK: libopenblas64_
  LIBM: libopenlibm
  LLVM: libLLVM-3.7.1 (ORCJIT, haswell)

[ScottPJones]

This should be closed, it is fixed on both Formatting.jl and Format.jl