Fix error when menu URL shorter than 4 characters is set

Fix the error when calling slicestr to slice out-of-range bounds

See #439

layouts/partials/navbar.html

@@ -51,11 +51,13 @@
         </li>
 
         {{ else }}
-
-        {{ if eq "http" (slicestr .URL 0 4) }}
-          {{ $.Scratch.Set "target" " target=\"_blank\" rel=\"noopener\"" }}
-        {{ else }}
-          {{ $.Scratch.Set "target" "" }}
+        
+        {{/* Set target for link. */}}
+        {{ $.Scratch.Set "target" "" }}
+        {{ if gt (len .URL) 4 }}
+          {{ if eq "http" (slicestr .URL 0 4) }}
+            {{ $.Scratch.Set "target" " target=\"_blank\" rel=\"noopener\"" }}
+          {{ end }}
         {{ end }}
 
         <li class="nav-item">