fix type instability/invalidations from nextind

[ranocha]

I found this while hunting some invalidations coming from ForwardDiff.jl

inserting promote_rule(::Type{R}, ::Type{ForwardDiff.Dual{T, V, N}}) where {R<:Real, T, V, N} in ForwardDiff at ~/.julia/packages/ForwardDiff/pDtsf/src/dual.jl:425 invalidated:
   backedges: 1: superseding promote_rule(::Type, ::Type) in Base at promotion.jl:310 with MethodInstance for promote_rule(::Type{Int64}, ::Type{S} where S<:Real) (1 children)
              2: superseding promote_rule(::Type, ::Type) in Base at promotion.jl:310 with MethodInstance for promote_rule(::Type{UInt8}, ::Type) (8 children)
              3: superseding promote_rule(::Type, ::Type) in Base at promotion.jl:310 with MethodInstance for promote_rule(::Type{Int64}, ::Type) (82 children)
   18 mt_cache

The output from ascend from SnoopCompile.jl showed that the output of nextind was inferred to Any but the documentation asserts that it should be an Int:

help?> nextind
search: nextind IndexCartesian MissingException current_exceptions InterruptException InvalidStateException

  nextind(str::AbstractString, i::Integer, n::Integer=1) -> Int

Note that the same type assertion is used in the line above the one I changed.