completely initialize SystemError (#44192)

(cherry picked from commit 2db86f2)

Author: sjkelly

base/io.jl

@@ -20,7 +20,7 @@ struct SystemError <: Exception
     extrainfo
     SystemError(p::AbstractString, e::Integer, extrainfo) = new(p, e, extrainfo)
     SystemError(p::AbstractString, e::Integer) = new(p, e, nothing)
-    SystemError(p::AbstractString) = new(p, Libc.errno())
+    SystemError(p::AbstractString) = new(p, Libc.errno(), nothing)
 end
 
 lock(::IO) = nothing

test/error.jl

@@ -81,3 +81,8 @@ end
     # non-Functions
     @test retry(Float64)(1) === 1.0
 end
+
+@testset "SystemError initialization" begin
+    e = SystemError("fail")
+    @test e.extrainfo === nothing
+end