00systemd: check if systemd version is a number

The recent systemd upstream introduced a slightly modified version
string which included information about a git commit, which however
broke the version check in dracut. Unfortunately, the (( )) bash syntax
went along with it in certain cases and introduced a pretty nasty issue,
when the systemd would boot up but with slightly changed environment.

To prevent this from happening in the future, let's at least check if
the version parsed from the `systemd --version` output is a comparable
number.

modules.d/00systemd/module-setup.sh

@@ -5,6 +5,11 @@ check() {
     [[ $mount_needs ]] && return 1
     if require_binaries $systemdutildir/systemd; then
         SYSTEMD_VERSION=$($systemdutildir/systemd --version | { read a b a; echo $b; })
+        # Check if the systemd version is a valid number
+        if ! [[ $SYSTEMD_VERSION =~ ^[0-9]+$ ]]; then
+            dfatal "systemd version is not a number ($SYSTEMD_VERSION)"
+            exit 1
+        fi
         (( $SYSTEMD_VERSION >= 198 )) && return 0
        return 255
     fi