Fix potential error when getting ID from a given index

Originally, if user gives command like 'gist .', _gist_id() still
returns true

1 files changed, 2 insertions, 2 deletions

diff --git a/gist b/gist
index 2b74732..ae9596a 100755
--- a/gist
+++ b/gist
@@ -406,8 +406,8 @@ _repo_url() {
 
 # Get gist id from index files
 _gist_id() {
-  GIST_ID=$( (grep -hs '' $INDEX || true) | sed -n -e "/^$1 / p" | cut -d' ' -f2 | sed -E -e 's#.*/##')
-  if [[ -z $GIST_ID ]]; then
+  GIST_ID=$(sed -n -e "/^$1 / p" $INDEX | cut -d' ' -f2 | sed -E -e 's#.*/##')
+  if [[ -z $GIST_ID || ! $1 =~ [0-9a-z]+ ]]; then
     echo -e "$(hint=false _show_list | sed -Ee 's/^( *[0-9a-z]+)/\\033[5m\1\\033[0m/')"
     echo
     echo -e "Not a valid index: \e[33m$1\e[0m"