check if completion script is readable

fixes #444

libexec/rbenv-init

@@ -83,11 +83,8 @@ if [[ ":${PATH}:" != *:"${RBENV_ROOT}/shims":* ]]; then
   echo 'export PATH="'${RBENV_ROOT}'/shims:${PATH}"'
 fi
 
-case "$shell" in
-bash | zsh )
-  echo "source \"$root/completions/rbenv.${shell}\""
-  ;;
-esac
+completion="${root}/completions/rbenv.${shell}"
+[ -r "$completion" ] && echo "source '$completion'"
 
 if [ -z "$no_rehash" ]; then
   echo 'rbenv rehash 2>/dev/null'

test/init.bats

@@ -22,7 +22,7 @@ load test_helper
   root="$(cd $BATS_TEST_DIRNAME/.. && pwd)"
   run rbenv-init -
   assert_success
-  assert_line 9;source "'${root}'/libexec/../completions/rbenv.bash"';
+  assert_line 4;source '${root}/libexec/../completions/rbenv.bash'";
 }
 
 @test "option to skip rehash" {