Fix `virtualenv-prefix` when the version is `system`

bin/pyenv-virtualenv-prefix

@@ -32,6 +32,10 @@ base_prefix() { # pyvenv
 
 VIRTUALENV_PREFIX_PATHS=()
 for version in "${versions[@]}"; do
+  if [ "$version" = "system" ]; then
+    echo "pyenv-virtualenv: version \`${version}' is not a virtualenv" 1>&2
+    exit 1
+  fi
   PREFIX="$(pyenv-prefix "${version}")"
   if [ -f "${PREFIX}/bin/activate" ]; then
     VIRTUALENV_PREFIX_PATH="$(real_prefix "${version}" || base_prefix "${version}" || true)"

test/virtualenv-prefix.bats

@@ -102,6 +102,19 @@ ${PYENV_ROOT}/versions/3.3.3:${PYENV_ROOT}/versions/3.4.0
 OUT
 }
 
+@test "should fail if the version is the system" {
+  stub pyenv-version-name "echo system"
+
+  PYENV_VERSION="system" run pyenv-virtualenv-prefix
+
+  unstub pyenv-version-name
+
+  assert_failure
+  assert_output <<OUT
+pyenv-virtualenv: version \`system' is not a virtualenv
+OUT
+}
+
 @test "should fail if the version is not a virtualenv" {
   stub pyenv-version-name "echo 3.4.0"
   stub pyenv-prefix "3.4.0 : echo \"${PYENV_ROOT}/versions/3.4.0\""