vim-patch.sh: fix "unbound variable" error with Bash < 4.4 [ci skip] (#10917)

Ref: https://github.com/neovim/neovim/pull/10888#issuecomment-526774032

scripts/vim-patch.sh

@@ -430,6 +430,7 @@ list_missing_vimpatches() {
       --shell)
   )"
   # Exit in case of errors from the above eval (empty vim_commit_tags).
+  if ! (( "${#vim_commit_tags[@]}" )); then
     msg_err "Could not get Vim commits/tags."
     exit 1
   fi
@@ -449,6 +450,7 @@ list_missing_vimpatches() {
       patch_number="vim-patch:${vim_tag:1}" # "v7.4.0001" => "7.4.0001"
       if [[ "${tokens[$patch_number]-}" ]]; then
         continue
+      fi
       echo "$vim_tag"
     else
       echo "$vim_commit"