Backport Fix for #8871

Mapreduce didn't work with `Range{BigInt}`, because the length wasn't a
`Int`, because of potential overflow. This patch just converts the
length to Int with a checked conversion, because a mapreduce on larger
arrays will probably never finish anyway.

Also fix some broken test of prod

(cherry picked from commit 9493465)

Fix test for 32 bit

The test added in #8893 shouldn't use typemax(Int), but typemax(Int64)

(cherry picked from commit 49083be)

base/reduce.jl

@@ -151,7 +151,7 @@ mr_empty(f, op::AndFun, T) = true
 mr_empty(f, op::OrFun, T) = false
 
 function _mapreduce{T}(f, op, A::AbstractArray{T})
-    n = length(A)
+    n = Int(length(A))
     if n == 0
         return mr_empty(f, op, T)
     elseif n == 1

test/reduce.jl

@@ -91,16 +91,19 @@ end
 
 # prod
 
-prod(Int[]) === 0
-prod(Int8[]) === 0
-prod(Float64[]) === 0.0
+@test prod(Int[]) === 1
+@test prod(Int8[]) === 1
+@test prod(Float64[]) === 1.0
 
-prod([3]) === 0
-prod([int8(3)]) === 0
-prod([3.0]) === 0.0
+@test prod([3]) === 3
+@test prod([int8(3)]) === 3
+@test prod([3.0]) === 3.0
 
-prod(z) === 120
-prod(fz) === 120.0
+@test prod(z) === 120
+@test prod(fz) === 120.0
+
+@test prod(1:big(16)) == big(20922789888000)
+@test prod(big(typemax(Int64)):big(typemax(Int64))+16) == BigInt("25300281663413827620486300433089141956148633919452440329174083959168114253708467653081909888307573358090001734956158476311046124934597861626299416732205795533726326734482449215730132757595422510465791525610410023802664753402501982524443370512346073948799084936298007821432734720004795146875180123558814648586972474376192000")
 
 # check type-stability
 prod2(itr) = invoke(prod, (Any,), itr)