admin/pin: Enforce that index is a number

Validate that we're parsing a number; we want to guard
against typos.

Closes: https://github.com/ostreedev/ostree/issues/2171

tests/test-admin-deploy-2.sh

@@ -26,7 +26,7 @@ set -euo pipefail
 # Exports OSTREE_SYSROOT so --sysroot not needed.
 setup_os_repository "archive" "syslinux"
 
-echo "1..7"
+echo "1..8"
 
 ${CMD_PREFIX} ostree --repo=sysroot/ostree/repo pull-local --remote=testos testos-repo testos/buildmaster/x86_64-runtime
 rev=$(${CMD_PREFIX} ostree --repo=sysroot/ostree/repo rev-parse testos/buildmaster/x86_64-runtime)
@@ -102,6 +102,13 @@ ${CMD_PREFIX} ostree admin pin -u 0
 assert_n_pinned 0
 echo "ok pin unpin"
 
+for p in medal 0medal '' 5000 9999999999999999999999999999999999999; do
+    if ${CMD_PREFIX} ostree admin pin ${p}; then
+        fatal "created invalid pin ${p}"
+    fi
+done
+echo "ok invalid pin"
+
 ${CMD_PREFIX} ostree admin pin 0 1
 assert_n_pinned 2
 assert_n_deployments 2