ssh_known_hosts_file: fix bug with empty list (fix #11)

We should return an empty file if there are no known hosts files
as this will cause no hosts to be considered known, which means
we're defaulting secure here.

src/ssh_options.jl

@@ -138,7 +138,8 @@ function ssh_known_hosts_file()
     for file in files
         ispath(file) && return file
     end
-    return files[1]
+    return !isempty(files) ? files[1] :
+        isfile("/dev/null") ? "/dev/null" : tempname()
 end
 
 ## helper functions

test/runtests.jl

@@ -109,6 +109,8 @@ end
         # empty
         ENV["SSH_KNOWN_HOSTS_FILES"] = ""
         @test ssh_known_hosts_files() == []
+        file = ssh_known_hosts_file()
+        @test !isfile(file) || isempty(read(file))
         # explicit default
         ENV["SSH_KNOWN_HOSTS_FILES"] = path_sep
         default = joinpath(homedir(), ".ssh", "known_hosts")