dracut.sh: harden host_modalias reading

Some weird PPC driver make their modulias unreadable

$ cat /sys/devices/vio/4000/modalias
cat: /sys/devices/vio/4000/modalias: No such device

dracut.sh

@@ -921,7 +921,7 @@ if [[ $hostonly ]]; then
     declare -A host_modalias
     find  /sys/devices/ -name modalias -print > "$initdir/.modalias"
     while read m; do
-        host_modalias["$(<"$m")"]=1
+        modalias="$(<"$m")" && [[ $modalias ]] && host_modalias["$modalias"]=1
     done < "$initdir/.modalias"
     rm -f -- "$initdir/.modalias"