Fix test failures

This fixes a panic due to integer overflow (0usize - 1) and bumps
up the default limit to fix test suite failures.

Author: DemiMarie

Diff for: html5ever/src/tree_builder/actions.rs

@@ -276,6 +276,9 @@ impl<Handle, Sink> TreeBuilderActions<Handle>
                     self.active_formatting.remove(fmt_elem_index);
                 }
             );
+            if fmt_elem_stack_index == 0 {
+                return
+            }
 
             // 7.
             if !self.in_scope(default_scope, |n| self.sink.same_node(&n, &fmt_elem)) {

Diff for: html5ever/src/tree_builder/mod.rs

@@ -59,7 +59,7 @@ impl<T> LimitedVec<T> {
     pub fn new(limit: usize) -> Self {
         LimitedVec {
             vec: vec![],
-            limit: if limit == 0 { 10 } else { limit },
+            limit: if limit == 0 { 20 } else { limit },
         }
     }
 
@@ -150,7 +150,7 @@ impl Default for TreeBuilderOpts {
             iframe_srcdoc: false,
             drop_doctype: false,
             ignore_missing_rules: false,
-            max_stack_depth: 10,
+            max_stack_depth: 20,
             quirks_mode: NoQuirks,
         }
     }