Fix #17956 and add test. The bug was introduced in 103db50

(cherry picked from commit b1e8216)
ref #18012

base/env.jl

@@ -94,7 +94,7 @@ end
 function next(hash::EnvHash, block::Tuple{Ptr{UInt16},Ptr{UInt16}})
     pos = block[1]
     blk = block[2]
-    len = ccall(:wcslen, UInt, (Ptr{UInt16},), pos)
+    len = ccall(:wcslen, UInt, (Ptr{UInt16},), pos) + 1
     buf = Array{UInt16}(len)
     unsafe_copy!(pointer(buf), pos, len)
     env = transcode(String, buf)

test/env.jl

@@ -30,3 +30,36 @@ key = randstring(25)
 @test !haskey(ENV,key)
 @test_throws KeyError ENV[key]
 @test get(ENV,key,"default") == "default"
+
+# Test for #17956
+@test length(ENV) > 1
+k1, k2 = "__test__", "__test1__"
+withenv(k1=>k1, k2=>k2) do
+    b_k1, b_k2 = false, false
+    for (k, v) in ENV
+        if k==k1
+            b_k1=true
+        elseif k==k2
+            b_k2=true
+        end
+    end
+    @test b_k1 && b_k2
+    io = IOBuffer()
+    show(io, ENV)
+    s = takebuf_string(io)
+    @test contains(s, "$k1=$k1")
+    @test contains(s, "$k2=$k2")
+
+    @test pop!(ENV, k1) == k1
+    @test !haskey(ENV, k1)
+    ENV[k1] = k1
+    @test pop!(ENV, k1) == k1
+    @test pop!(ENV, k1, "not_there") == "not_there"
+
+    ENV[k1] = k1
+    @test delete!(ENV, k1) == ENV
+    @test !haskey(ENV, k1)
+end
+
+# Test for #10853
+@test withenv(Dict{Any,Any}()...) do; true; end